The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

protohippus
    n 1: Pliocene horse approaching donkeys in size