orohippus

from The Century Dictionary.

• noun A genus of fossil horses, of the family Equidæ, based upon remains from the Eocene of North America, having four toes on the fore feet and three on the hind feet. There are several species, all of very small size, only about as large as a fox.
• noun [lowercase] A species of the above genus.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Paleon.) A genus of American Eocene mammals allied to the horse, but having four toes in front and three behind.