Horn (animal anatomy), one of the hard protuberances that grows from the foreheads or snouts of various hoofed mammals, such as rhinoceroses, deer, giraffes, goats, and cattle. Horns are generally curved and pointed and serve chiefly as weapons of defense or attack; in some species, only the male is horned. All horns have in common an essential substance called keratin, a fibrous protein produced in the outer layer of skin; keratin is also a component of beaks, hair, nails, hooves, scales, shells, claws, and feathers.

The horns or antlers of deer are solid bone. Those of giraffes are also of bone but are covered by skin. The horn of the rhinoceros is a hollow cone of compacted, hairlike structures embedded in skin. The pronghorn of the western U.S. has a permanent horny core covered by a sheath of horn that is annually shed. Horn is used in making ornamental articles and musical instruments. The term horn is applied to analogous appendages in many insects.