The Collaborative International Dictionary of English v.0.48:

Pliohippus \Pli`o*hip"pus\, n. [NL., fr. E. pliocene + Gr. ?
   horse.] (Paleon.)
   An extinct genus of horses from the Pliocene deposits. Each
   foot had a single toe (or hoof), as in the common horse.
   [1913 Webster]