Leather is dropped by cows. Also crafted with four rabbit hides. Each cow could drop one or two pieces of leather. Leather is the 1st tier armor in Mine Blocks.
As a Crafting Recipe
It can be made into armor. To make Leather chest plates, you need to fill out all the squares in the crafting area, but take out one piece of leather on the top middle square in the first row. To make leather boots, you need to put two pieces of on top of each other, then do it on the other side. To make leather pants, you need to put three pieces of leather on each two ends on top of each other, then put one piece of leather in the top middle between the two rows of leather. To make Leather Helmet, you need to put three pieces of leather in the first two squares on each end on top of each other, then put one piece of leather on the top middle between the two rows of leather. You need 8 pieces of leather for a chest-plate, 4 for boots, 7 for pants, and 5 for a helmet.