hoof

Horny covering that protects the sensitive parts of the foot of an animal. The possession of hooves is characteristic of the orders Artiodactyla (even-toed ungulates such as deer and cattle), and Perissodactyla (horses, tapirs, and rhinoceroses).

The flexibility of the hoof is promoted by a fluid secreted by the keratogenous (horn-producing) membrane. The cloven hoof of the Artiodactyla has been evolved for walking and climbing on irregular surfaces by the formation of a separate hoof on each digit of the foot. Horses walk on the third digit of the foot, while cattle walk on the third and fourth.