The odd-toed ungulates (Perissodactyla, from Greek perissos, meaning odd, and daktylos, meaning finger or toe) are ungulates (hoofed animals) whose weight is borne primarily on their third toe.

In contrast with even-toed ungulates, (bison, cows, goats, pigs, sheep and other animals) which have one or more stomach chambers, from which they ruminate (regurgitate and re-chew their food), the three families of the odd-toed ungulates are characterized by somewhat simple stomachs and then digest plant cellulose in their intestines, a process called hindgut fermentation.

The odd-toed ungulates also have molar teeth with transverse ridges on the grinding surface, and posterior premolars resembling true molars.

The Perissodactyla appear relatively abruptly in both North America and Asia at the beginning of the lower Paleocene epoch, just before 63 million years ago.

The Order Perissodactyla is composed of three families which are:

(1) Equidae [asses, horses, zebras, 1 genus, 7 species];

(2) Rhinoceritidae [rhinoceroses, 4 genera, 4 species];

(3) Tapiridae [tapirs, 1 genus, 5 species].

In the family of Equidae, Horses in listed in the subspecies caballus, which are domesticated, although some domesticated populations live in the wild as feral horses. These feral populations are not true wild horses, as this term, feral is used to describe horses that have never been domesticated, such as the endangered Przewalski′s horse, (Mongolian wild horse or takhi), a separate subspecies, and the only remaining true wild horse.