The Collaborative International Dictionary of English v.0.48:

eohippus \eo*hip"pus\ ([=e]`[-o]*h[i^]p"p[u^]s), n. [Gr. 'hw`s
   dawn + "ipposhorse.]
   an extinct primitive dog-sized 4-toed Eocene mammal, the
   earliest horse known in the line of descent of the modern
   horse. It is classed in the extinct genus Hydracotherium.
   Called also dawn horse.

   Syn: dawn horse.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

dawn horse
    n 1: earliest horse; extinct primitive dog-sized four-toed
         Eocene animal [syn: eohippus, dawn horse]