Hoof
Hoof, the curved, horny covering that protects the toes of ungulates (grazing mammals). The hoof is a growth of epidermis (outer skin) that corresponds to a nail or claw. But unlike a nail or claw, the hoof is a blunt, thickened growth that almost completely encloses the toes or foot.
Hoofed animals may have an odd or even number of toes. The horse, ass, and zebra have a single toe on each foot. Most ungulates, including the pig, sheep, and deer, have four toes. In these animals, the two middle toes are developed to considerable size to form a cleft, or divided, hoof.