Real Dictionary

eohippus in American English
an extinct progenitor (genus Hyracotherium) of the modern horse, found in the Lower Eocene of W U.S.: it was about the size of a fox and had four toes on the front feet and three on the hind
noun: the earliest known horse, a member of the extinct genus Hyracotherium (Eohippus), from the early Eocene Epoch of the Northern Hemisphere; a terrier-sized herbivore having four hoofed toes on each forefoot and three on each hind foot, and low-crowned teeth

eohippus in British English
noun: the earliest horse: an extinct Eocene dog-sized animal of the genus with four-toed forelegs, three-toed hindlegs, and teeth specialized for browsing