Why are horses, tapirs, and rhinoceroses grouped together as perissodactyls?

Horses, tapirs, and rhinoceroses are grouped together as perissodactyls (order Perissodactyla) primarily because they are odd-toed ungulates, characterized by having either one or three hoofed toes on each hindfoot.12

The term "perissodactyl" is derived from the Greek words perissos ("odd") and daktylos ("finger"), a classification established to distinguish these animals from the even-toed artiodactyls.1

Beyond their toe count, these mammals share a specific anatomical trait known as the mesaxonic condition. In perissodactyls, the axis of symmetry of the limb passes through the third or middle toe, which is the most strongly developed and bears the majority of the animal's weight. This contrasts with the paraxonic condition found in artiodactyls, where the weight-bearing axis passes between the third and fourth toes. Additionally, perissodactyls possess a simple, undivided stomach and rely on a large cecum and colon for the fermentation of fibrous plant material.345