The leg bone is an item that can be dropped by many prehistoric creatures upon death or by mining fossil blocks. There is an individual leg bone item for each creature that drops it, but the item texture is always the same.
Mammals, birds, coelacanths, and nautilus do not drop leg bones. The leg bone is used to craft shin guards, a type of skull armor. Any species of leg bone will work in the crafting recipe, along with vanilla Minecraft bones. However, they cannot create bonemeal and cannot be analyzed to produce DNA.
Only nautilus and the three fish are lacking leg bones (understandably). They can be analyzed to get DNA of the same species in this update as well as used to craft skull shin guards, as seen below. They still cannot produce bonemeal, though.