Source : Webster's Revised Unabridged Dictionary (1913)

Mesohippus \Mes`o*hip"pus\, n. [NL., fr. Gr. me`sos middle + ? a
   horse.] (Paleon.)
   An extinct mammal of the Horse family, but not larger than a
   sheep, and having three toes on each foot.

Source : WordNet®

mesohippus
     n : North American 3-toed Oligocene animal; probably not
         directly ancestral to modern horses