A horn is a pointed projection of the skin on the head consisting of a covering horn (keratin and other proteins) surrounding a core of living bone.
Many animal 'horns' are made of a-keratin which is the same stuff that forms hair, claws and fingernails. Strictly speaking these are not classed as horns. As these keratin cells die they harden and...
elepant
calpol
It is primarily as a form of defense. It is also a form of show and strength for some species during mating season.