The Collaborative International Dictionary of English v.0.48:

Miohippus \Mi`o*hip"pus\ (m[imac]`[-o]*h[i^]p"p[u^]s), n. [NL.,
   fr. Gr. mei`wn less + "ippo`s horse.] (Paleon.)
   An extinct Miocene mammal of the Horse family, closely
   related to the genus Anhithecrium, and having three usable
   hoofs on each foot.
   [1913 Webster]