Rhinoceroses are usually divided into three families. The first , and most primitive, is the family Hyracodon. Hyracodonts are also called the "running rhinoceroses" due to the fact that the species belonging to this family display longer, slimmer legs than the usual or typical rhinoceros. This type of leg structure would have been beneficial to these animals who were small and most likely needed to defend themselves against larger predators. Hyracodonts are found from the Oligocene to the late Eocene in North America and Asia.

A Hyracodont possessed long slim legs which aided the animal from running from predators. .