The early horse genus, about the size of a small dog and known from Eocene fossils in North America, is Eohippus.
Eohippus was a tiny early equid that lived during the Early Eocene, roughly 56 to 47 million years ago. Its remains are known from North America, and the name means “dawn horse,” reflecting its place in the traditional story of horse evolution.
It looked very different from a modern horse. Eohippus stood about three hands high, had four toes on each front foot and three on each hind foot, and carried weight on padded feet rather than a single large hoof. Its teeth already showed some features associated with later horses, although it also had relatively large canine teeth.
The name has a complicated taxonomic history. Fossils once placed in Eohippus were often called Hyracotherium, and some classifications still discuss the animals under that older name. Mesohippus was a later, larger three-toed horse, while Equus is the modern horse genus; neither is the best answer to this question.