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