Dry Bones

Dry Bones are the undead skeletons of Koopa Troopas that now can walk again. They are immune to fireballs, so you must stomp them. But instead of squishing flat or going into its shell, Dry Bones breaks apart, its bones scattering. But after a few seconds, it will (literally) pull itself back together again. Dry Bones usually only appear in castle or fortress levels in the games they are include in.