The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mesohippus
    n 1: North American three-toed Oligocene animal; probably not
         directly ancestral to modern horses