The horse is the “proudest conquest of Man,” according to the French zoologist Le Comte de Buffon. Its place was at its master's side in the graves of the Scythian kings or in the tombs of the pharaohs. Many early human cultures were centred on possession of the horse. Superstition read meaning into the colours of the horse, and a horse's head suspended near a grave or sanctuary or on the gables of a house conferred supernatural powers on the place. Greek mythology created the centaur, the most obvious symbol of the oneness of horse and rider. White stallions were the supreme sacrifice to the gods, and the Greek general Xenophon recorded that “Gods and heroes are depicted on well-trained horses.” A beautiful and well-trained horse was, therefore, a status symbol in ancient Greece. Kings, generals, and statesmen, of necessity, had to be horsemen. The names of famous horses are inseparably linked to those of their famous riders: Bucephalus, the charger of Alexander the Great; Incitatus, once believed to have been made a senator by the Roman emperor Caligula (see Researcher's Note); El Morzillo, Cortés's favourite horse, to whom the Indians erected a statue; Roan Barbery, the stallion of Richard II, mentioned by Shakespeare; Copenhagen, the Duke of Wellington's horse, which was buried with military honours.

The horse in life has served its master in travels, wars, and labours and in death has provided many commodities. Long before their domestication horses were hunted by primitive tribes for their flesh, and horsemeat is still consumed by people in parts of Europe and in Iceland and is the basis of many pet foods. Horse bones and cartilage are used to make glue. Tetanus antitoxin is obtained from the blood serum of horses previously inoculated with tetanus toxoid. From horsehide a number of articles are manufactured, including fine shoes and belts. The cordovan leather fabricated by the Moors in Córdoba, Spain, was originally made from horsehide. Stylish fur coats are made of the sleek coats of foals. Horsehair has wide use in upholstery, mattresses, and stiff lining for coats and suits; high-quality horsehair, usually white, is employed for violin bows. Horse manure, which today provides the basis for cultivation of mushrooms, was used by the Scythians for fuel. Mare's milk was drunk by the Scythians, the Mongols, and the Arabs.

The primitive horse probably stood 12 hands (about 120 cm, or 48 inches) tall at the withers, the high point on the back at the base of the neck, and was dun coloured. Domestic horses gone wild, such as the mustangs of western North America, tend to revert to those primitive features under random mating; they generally are somewhat taller (about 15 hands), usually gray, dun, or brownish in colour, and move in herds led by a stallion.

The horse's general form is characteristic of an animal of speed: the long leg bones (skeleton) pivot on pulley-like joints that restrict movement to the fore and aft, the limbs are levered to muscle masses in such a way as to provide the most efficient use of energy, and the compact body is supported permanently on the tips of the toes, allowing fuller extension of the limbs in running.

The rounded skull houses a large and complex brain, well developed in those areas that direct muscle coordination. While the horse is intelligent among subhuman animals, it is safe to say that the horse is more concerned with the functioning of its acute sensory reception and its musculature than with mental processes. Though much has been written about “educated” horses that appear to exhibit an ability to spell and count, it is generally agreed that in such cases a very perceptive animal is responding to cues from its master. But this ability is remarkable enough in its own right—for the cues are often given unconsciously by the human trainer, and detection of such subtle signals requires extremely sharp perception.

Under domestication the horse has diversified into three major types, based on size and build: draft horses, heavy limbed and up to 20 hands (200 cm, or 80 inches) high; ponies, by convention horses under 14.2 hands (about 147 cm, or 58 inches) high; and light horses—the saddle or riding horses—which fall in the intermediate size range. Domestic horses tend to be nearsighted, less hardy than their ancestors, and often high-strung, especially thoroughbreds, where intensive breeding has been focused upon speed to the exclusion of other qualities. The stomach is relatively small, and, since much vegetation must be ingested to maintain vital processes, foraging is almost constant under natural conditions. Domestic animals are fed several (at least three) times a day in quantities governed by the exertion of the horse.

The extremely large eyes (eye, human) placed far back on the elongated head admirably suit the horse for its chief mode of defense: flight. Its long neck and high-set eyes, which register a much wider range than do the eyes of a human being, enable the horse to discern a possible threat even while eating low grasses. Like human vision, the horse's vision is binocular, but only in the narrow area directly forward, and evidence suggests that it does not register colour. While visual acuity is high, the eyes do not have variable focus, and objects at different distances register only on different areas of the retina, which requires tilting movements of the head. The senses of smell and hearing seem to be keener than in human beings. As the biologist George Gaylord Simpson (Simpson, George Gaylord) put it in Horses (1961):

From the dun of the primitive horse has sprung a variety of colours and patterns, some highly variable and difficult to distinguish. Among the most important colours are black, bay, chestnut (and sorrel), palomino, cream, and white.

The horse's natural food is grass. For stabled horses, the diet generally consists of hay and grain. The animal should not be fed immediately before or after work, to avoid digestive problems. Fresh water is important, especially when the horse is shedding its winter coat, but the animal should never be watered when it is overheated after working. Oats provide the greatest nutritional value and are given especially to foals. Older horses, whose teeth are worn down, or those with digestive troubles, can be provided with crushed oats. Chaff (minced straw) can be added to the oat ration of animals that eat greedily or do not chew the grain properly. Crushed barley is sometimes substituted in part for oats. Hay provides the bulk of the horse's ration and may be of varying composition according to locale. Mash is bran mixed with water and with various invigorating additions or medications. It may be given to horses with digestive troubles or deficient eating habits. Corn (maize) is used as a fattening cereal, but it makes the horse sweat easily. Salt is needed by the horse at all times and especially when shedding. Bread, carrots, and sugar are tidbits often used to reward an animal by the rider or trainer. In times of poverty horses have adapted to all sorts of food—potatoes, beans, green leaves, and in Iceland even fish—but such foods are not generally taken if other fare is available. A number of commercial feed mixes are available to modern breeders and owners; these mixes contain minerals, vitamins, and other nutrients and are designed to provide a balanced diet when supplemented with hay.

The horse's nervous system is highly developed and gives proof to varying degrees of the essential faculties that are the basis of intelligence: instinct, memory, and judgment. Foals, which stand on their feet a short while after birth and are able to follow their mothers within a few hours, even at this early stage in life exhibit the traits generally ascribed to horses. They have a tendency to flee danger. They express fear sometimes by showing panic and sometimes by immobility. Horses rarely attack and do so either when flight is impossible or when driven to assault a person who has treated them brutally.

Habit governs a large number of their reactions. Instinct, together with a fine sense of smell and hearing, enables them to sense water, fire, even distant danger. An extremely well-developed sense of direction permits the horse to find its way back to its stables even at night or after a prolonged absence. The visual memory of the horse prompts it to shy repeatedly from an object or place where it had earlier experienced fear. The animal's auditory memory, which enabled ancient army horses or hunters to follow the sounds of the bugles, is used in training. When teaching, the instructor always uses the same words and the same tone of voice for a given desired reaction. Intelligent horses soon attach certain movements desired by their trainers to particular sounds and even try to anticipate their rider's wishes.

While instinct is an unconscious reaction more or less present in all individuals of the same species, the degree of its expression varies according to the individual and its development. Most horses can sense a rider's uncertainty, nervousness, or fear and are thereby encouraged to disregard or even deliberately disobey the rider. Highbred animals, which give evidence of greater intelligence than those of low breeding, are capable not only of acts of vengeance and jealousy against their riders but also of expressions of confidence, obedience, affection, and fidelity. They are less willing than a lowbred horse to suffer rough handling or unjust treatment.

Cunning animals have been known to employ their intelligence and physical skill to a determined end, such as opening the latch of a stall or the lid of a chest of oats.

The onset of adult sex characteristics generally begins at the age of 16 to 18 months; the horse is considered mature, according to the breed, at approximately three years and adult at five. Fecundity varies according to the breed and may last beyond age 20 with thoroughbreds and to 12 or 15 with other horses. The gestation period is 11 months, 280 days being the minimum in which the foal can be born with expectation to live. As a rule a mare produces one foal per mating, twins occasionally, and triplets rarely. The foal is weaned at six months.

The useful life of a horse varies according to the amount of work it is required to do and the maintenance furnished by its owner. A horse that is trained carefully and slowly and is given the necessary time for development may be expected to serve to an older age than a horse that is rushed in its training. Racehorses that enter into races at the age of two rarely remain on the turf beyond eight. Well-kept riding horses, on the contrary, may be used more than 20 years.

The life span of a horse is calculated at six to seven times the time necessary for his physical and mental development; that is, 30 to 35 years at the utmost, the rule being about 20 to 25 years. Ponies generally live longer than larger horses. There are a number of examples of horses that have passed the usual limit of age. The veterinary university of Vienna conserves the skeleton of a thoroughbred mare of 44 years of age. There have been reports made of horses living to the early 60s in age.

Horses are subjected to a number of contagious diseases, such as influenza, strangles, glanders, equine encephalomyelitis, and swamp fever. Their skin is affected by parasites, including certain mites, ticks, and lice. Those with sensitive skin are especially subject to eczemas and abscesses, which may result from neglect or contamination. Sores caused by injuries to the skin from ill-fitting or unclean saddles and bridles are common ailments. The horse's digestive tract is particularly sensitive to spoiled feed, which causes acute or chronic indigestion, especially in hot weather. Worms can develop in the intestine and include the larvae of the botfly, pinworms, tapeworms, and roundworms (ascarids). Overwork and neglect may predispose the horse to pneumonia and rheumatism. The ailment known as roaring is an infection of the larynx that makes the horse inhale noisily; a milder form causes the horse to whistle. Chronic asthma, or “broken wind,” is an ailment that is considered to be all but incurable. A horse's legs and feet are sensitive to blows, sprains, and overwork, especially if the horse is young or is worked on hard surfaces. Lameness may be caused by bony growths, such as splints, spavins, and ringbones, by soft-tissue enlargements, known as windgalls, thoroughpins, and shoe boils, and by injury to the hooves, including sand crack, split hoof, tread thrush, and acute or chronic laminitis.

The first intensively domesticated horses were developed in Central Asia. They were small, lightweight, and stocky. In time, two general groups of horses emerged: the southerly Arab-Barb types (from the Barbary coast) and the northerly, so-called cold-blooded types. When, where, and how these horses appeared is disputed. Nevertheless, all modern breeds—the light, fast, spirited breeds typified by the modern Arabian, the heavier, slower, and calmer working breeds typified by the Belgian, and the intermediate breeds typified by the Thoroughbred—may be classified according to where they originated (e.g., Percheron, Clydesdale, and Arabian), by the principal use of the horse (riding, draft, coach horse), and by their outward appearance and size (light, heavy, pony).

The Anglo-Arab breed originated in France with a crossing of English Thoroughbreds to pure Arabians. The matings produced a horse larger than the Arabian and smaller than the Thoroughbred, of easy maintenance, and capable of carrying considerable weight in the saddle. Its coat is generally chestnut or bay.

The Cleveland Bay carriage horse, up to 17 hands high and generally bay in colour, is similar to the Yorkshire Coach horse. Both breeds are now used for the sport of driving.

The line leading from Eohippus to the modern horse exhibits the following evolutionary trends: increase in size, reduction in the number of hooves, loss of the foot pads, lengthening of the legs, fusion of the independent bones of the lower legs, elongation of the muzzle, increase in the size and complexity of the brain, and the development of crested, high-crowned teeth suited to grazing. This is not to imply that there was a steady, gradual progression in these characteristics leading inevitably from those of Eohippus to those of the modern horse. Some of these features, such as grazing dentition, appear abruptly in the fossil record, rather than as the culmination of numerous, gradual changes. Eohippus, moreover, gave rise to many now-extinct branches of the horse family, some of which differed substantially from the line leading to the modern equines.

Although Eohippus fossils occur in both the Old and New worlds, the subsequent evolution of the horse took place chiefly in North America. During the remainder of the Eocene, the prime evolutionary changes were in dentition. Orohippus, a genus from the Middle Eocene, and Epihippus, a genus from the Late Eocene, resembled Eohippus in size and in the structure of the limbs. But the form of the cheek teeth—the four premolars and the three molars found in each half of both jaws—had changed somewhat. In Eohippus the premolars and molars were clearly distinct, the molars being larger. In Orohippus the fourth premolar had become similar to the molars, and in Epihippus both the third and fourth premolars had become molarlike. In addition, the individual cusps that characterized the cheek teeth of Eohippus had given way in Epihippus to a system of continuous crests or ridges running the length of the molars and molariform premolars. These changes, which represented adaptations to a more specialized browsing diet, were retained by all subsequent ancestors of the modern horse.

Fossils of Mesohippus, the next important ancestor of the modern horse, are found in the Early and Middle Oligocene of North America (the Oligocene Epoch lasted from about 33.7 to 23.8 million years ago). Mesohippus was far more horselike than its Eocene ancestors—it was larger (averaging about six hands high); the snout was more muzzlelike; and the legs were longer and more slender. Mesohippus also had a larger brain. The fourth toe on the forefoot had been reduced to a vestige, so that both the forefeet and hindfeet carried three functional toes and a foot pad. The teeth remained adapted to browsing.

By the Late Oligocene, Mesohippus had evolved into a somewhat larger form known as Miohippus. The descendants of Miohippus split into various evolutionary branches during the Early Miocene (the Miocene Epoch lasted from about 23.8 to 5.3 million years ago). One of these branches, known as the anchitheres, included a variety of three-toed, browsing horses comprising several genera. Anchitheres were successful, and some genera spread from North America across the Bering land bridge into Eurasia.

It was a different branch, however, that led from Miohippus to the modern horse. The first representative of this line, Parahippus, appeared in the Early Miocene. Parahippus and its descendants marked a radical departure in that they had teeth adapted to eating grass. Grasses were at this time becoming widespread across the North American plains, providing Parahippus with a vast food supply. Grass is a much coarser food than succulent leaves and requires a different kind of tooth structure. The cheek teeth developed larger, stronger crests and became adapted to the side-to-side motion of the lower jaw necessary to grind grass blades. Each tooth also had an extremely long crown, most of which, in the young animal, was buried beneath the gum line. As grinding wore down the exposed surface, some of the buried crown grew out. This high-crowned tooth structure assured the animal of having an adequate grinding surface throughout its normal life span. Adaptations in the digestive tract must have occurred as well, but the organs of digestion are not preserved in the fossil record.

The change from browsing to grazing dentition was essentially completed in Merychippus, which evolved from Parahippus during the Middle and Late Miocene. Merychippus must have looked much like a modern pony. It was fairly large, standing about 10 hands high, and its skull was similar to that of the modern horse. The long bones of the lower leg had become fused; this structure, which has been preserved in all modern equines, is an adaptation for swift running. The feet remained three-toed, but in many species the foot pad was lost, and the two side toes became rather small. In these forms, the large central toe bore the animal's weight. Strong ligaments attached this hoofed central toe to the bones of the ankles and lower leg, providing a spring mechanism that pushed the flexed hoof forward after the impact of hitting the ground. Merychippus gave rise to numerous evolutionary lines during the Late Miocene. Most of these, including Hipparion, Neohipparion, and Nannippus, retained the three-toed foot of their ancestors. One line, however, led to the one-toed Pliohippus, the direct predecessor of Equus. Pliohippus fossils occur in the Early to Middle Pliocene beds of North America (the Pliocene Epoch lasted from about 5.3 to 1.8 million years ago).

Equus—the genus to which all modern equines, including horses, asses, and zebras, belong—evolved from Pliohippus toward the end of the Pliocene. Equus shows even greater development of the spring mechanism in the foot and exhibits straighter and longer cheek teeth. This new form was extremely successful and had spread from the plains of North America to South America and to all parts of the Old World by the Early Pleistocene (the Pleistocene Epoch lasted from about 1,800,000 to 10,000 years ago). Equus flourished in its North American homeland throughout the Pleistocene; then, about 10,000 to 8,000 years ago, Equus disappeared from North and South America. Scholars have offered various explanations for this disappearance, including the emergence of devastating diseases or the arrival of human populations (which presumably hunted the horse for food). Despite these speculations, the reasons for the demise of Equus in the New World remain uncertain. The submergence of the Bering land bridge prevented any return migration of horses from Asia, and Equus was not reintroduced into its native continent until the Spanish explorers brought horses in the early 16th century.

During the Pleistocene the evolution of Equus in the Old World gave rise to all the modern members of the genus. The modern horse, Equus caballus, became widespread from central Asia to most of Europe. Local types of horses, all breeds of this single species, undoubtedly developed, and three of these— Przewalski's horse from central Asia, the tarpan from eastern Europe and the Ukrainian steppes, and the forest horse of northern Europe—are generally credited as being the ancestral stock of the domestic horse. According to this line of thinking, Przewalski's horse and the tarpan formed the basic breeding stock from which the southerly “warm-blooded” horses developed, while the forest horse gave rise to the heavy, “cold-blooded” breeds.