1) Equus caballus 2) Horse 
Protohippus is an extinct three-toed genus of horse. It was roughly the size of a modern donkey. Fossil evidence suggests that it lived during the Late Miocene and Early Pliocene, from about 14 Ma to 6 Ma. Analysis of Protohippus` skull and teeth suggests that it is most closely related to the genus Calippus. Species include: ...
• (n.) A genus of fossil horses from the Lower Pliocene. They had three toes on each foot, the lateral ones being small.
Pro`to·hip'pus noun [ New Latin , from Greek ... first + ... horse.] (Paleon.) A genus of fossil horses from the Lower Pliocene. They had three toes on each foot, the lateral ones being small. 
[n] - Pliocene horse approaching donkeys in size
Protohippus Related horse-word units: caval-; equ-, equi-.
noun Pliocene horse approaching donkeys in size