Many groups of "modern" animals made their first appearance in the eocene. These include the first members of the orders of elephants, bats, whales, even-toed hooved animals (artiodactyls) and the odd-toed perissodactyls. The first horse, Hyracotherium (aka eohippus), appeared in the eocene. It is the earliest member of the family equidae, which includes all extinct horses and the modern genus Equus.

At left, a detail of a painting by Jay Matternes shows Eocene mammals. Although we can begin to recognize certain types of mammals, such as carnivores and perissodactyls, they barely resemble their modern counterparts.