eohippus

Hyracotherium.

[New Latin : EO– + Greek hippos, horse.]

The oldest known ancestor of the horse. A short, 25–50 cm high, rodent-like animal with four toes on each forefoot and three on each hind. Each toe had a hoof. The Eohippus survived by its speed. See also mesohippus.

The noun has one meaning:

Meaning #1: earliest horse; extinct primitive dog-sized 4-toed Eocene animal
  Synonym: dawn horse