genus of extinct early and middle Oligocene horses (the Oligocene Epoch occurred from 36.6 to 23.7 million years ago) commonly found as fossils in the rocks of the Badlands region of South Dakota, U.S. Mesohippus was the first of the three-toed horses and, although only the size of a modern collie dog, was very horselike in appearance. Mesohippus was still a browsing form; its teeth were unsuited to the grazing adopted by later, more advanced horses. Mesohippus gave rise to the next stage in horse evolution, the genus Miohippus, a larger form that was common in the Late Oligocene (30 to 23.7 million years ago).