Skeleton Bowser

Skeleton Bowser (A.K.A Dry Bowser, Skeletal Bowser, Bone Bowser, Bowser Bones) is just Bowser in the form of bones. Mario drops him into the lava. He could have appeared in the previous games but spriting resources weren't advanced enough to create him. Bowser Jr. revives him in some kind of magic pot on the last level and you have to defeat him again. It is possible that Dry Bones are created the same way.