Leather belongs to the class of polymers. Leather is made of collagen (protein), a natural polymer that is very similar to polyamide.
Rawhide is turned into leather by the tanning proces. This process crosslinks the polymer chains and increases the materials strength.
Leather is made from skins of goats, cows, snakes, alligators, sharks, salmons, camels and many others. The usual trade unit is the quare feet.