The Collaborative International Dictionary of English v.0.48:

Orohippus \Or`o*hip"pus\, n. [NL., fr. Gr. 'o`ros, mountain
   (referring to the Rocky Mountain region) + "i`ppos horse.]
   (Paleon.)
   A genus of American Eocene mammals allied to the horse, but
   having four toes in front and three behind.
   [1913 Webster]