Those found in Old World pigs are large and massive with large upper canines that grow outward and backwards forming tusks. The skull of the New World equivalent of a pig, the peccary, is small and more delicate. Its canines grow downward and meet with the lower canines to form precision scissors. Animals such as antelopes, goats, bison, sheep and cattle do not shed their horns. The one exception is the pronghorn which sheds its horns annually.
Horns are permanent and continue to grow throughout the animal's lifetime.
Horns are grown by all adult bovid males and by females of most genera. Your fingernails and hair are also made of keratin.