genus of extinct horses (horse) that originated in North America during the Late Eocene Epoch (37.2–33.9 million years ago). Miohippus evolved from the earlier genus Mesohippus; however, the former was larger and had a more-derived dentition than the latter. The number of toes in Miohippus was reduced to three, which enabled it to run considerably faster than its five-toed ancestors. Miohippus persisted into the Miocene Epoch (23–5.3 million years ago) side by side with its more horselike one-toed relatives.