Dry Bones

Dry Bones, or Skeleton Koopas, are recurring enemies in the Mario franchise, being the undead skeletal versions of Koopa Troopas and first appearing in Super Mario Bros. 3.

As they are already dead, killing/defeating them is easier said then done, as stomping on them only dismantles their bones before they resemble themselves and continue their patrols. They are also immune to fireballs.