Bones
Bones are all that remain after carcass decay. They can either be leg bones or rib-cages. Bones never despawn until the server crashes or resets, so massive boneyards can form as a result. A recently killed animal will have red bones stained with blood, but over time they turn white.
In the future, when an animal dies they will leave an entire skeleton, including a skull.
Boneyards
Boneyards naturally form as many animals die or get killed in one area. They form from the bones of dead animals. In future maps, there will also be permanent boneyards that are part of the map.