Source : Webster's Revised Unabridged Dictionary (1913)

Protohippus \Pro`to*hip"pus\, n. [NL., from Gr. ? first + ?
   horse.] (Paleon.)
   A genus of fossil horses from the Lower Pliocene. They had
   three toes on each foot, the lateral ones being small.

Source : WordNet®

protohippus
     n : Pliocene horse approaching donkeys in size