The Mûmak Skeleton is a generated structure found in Near Harad.
The skeleton is in the shape of one of the great Mûmakil of Harad (also known as Oliphaunts). It is made of bone blocks. It is believed that, due to dialogue with the Taurethrim, that the Mûmakil came from Far Harad and were 'stolen' by the Near Haradrim.
These skeletons are a good way of quickly getting bone meal, (assuming you happen to stumble across one) as you can get 968 bonemeal from the full skeleton of 121 bone blocks.