A Bone Thwomp in the Pump Works.
A Bone Thwomp is a species of Thwomp that acts as a bone in Bowser's Body in the game. They appear throughout the Pump Works. Bone Thwomps are more of an obstacle than an enemy, blocking the Mario Bros.'s path. If Bowser drinks water, Bone Thwomps will float, allowing the plumbers to proceed. This enemy reappears later in the Airway, but they are shorter, and plumper. They are used as stepping stones to access higher ground.