A small, herbivorous, extinct mammal of the genus Hyracotherium(or Eohippus)from the Eocene Epoch of the western United States, having four - toed front feet and three - toed hind feet and related ancestrally to the horse.

• 始祖马始新世产于美国西部的一种属于蹄兔（或始祖马）属的哺乳动物，已灭绝，体型小，食草，前蹄四趾，后蹄三趾，为马的远祖