Dry Bones is a skeleton version of Koopa. The way to beat them is to stand on them and then they fall apart. But don't think that they stay in little pieces, beacause they will come back to their original form! (as a skeleton that is)
Dry Bones' first appearance is in Super Mario Bros. 3, but he can also selected as an unlockable character in Mario Kart DS and Mario Party 7.