Skeleton of Mesohippus bairdi
The Mesohippus lived in the late Eocene to early Ogliocene eras (30-40 million years ago), in North America. These proto-horses had three toes, and primarily used the center toe. Though they were structurally a long way off from the modern horse (not to mention only 60 cm/~2 ft tall), they were much closer to the Equus ferus we know than their ancestors, the Hyracotherium. Aside from their feet, their teeth and legs were moving quickly up the the evolutionary ladder.
Notes on the Osteology of the White River Horses. Marcus S. Farr, 1896.