Bony Beetle

A Bony Beetle is an undead skeletal version of a Buzzy Beetle that has made minor appearances throughout the Mario series.

Super Mario World
They first appeared in Super Mario World and and its remake, where they would walk around, then stop and stick out spikes. They can be defeated with the swing of a cape, but are immune to Fireballs, so timing is critical. If Mario stomps one, it will fall apart and reassemble seconds later, similar to a Dry Bones. Due to graphical limitations, Bony Beetles appear to turn into Dry Bones whenever they collapse. Bony Beetles were first seen in the depths of Vanilla Fortress.

Paper Mario
They are later seen in Paper Mario in an underground section of Bowser's Castle (and several other places), with a complete makeover. They're defeated basically the same way as Dry Bones, but harder, so the player should whack them with the Hammer often when fighting them. They would engage battle with Mario in different ways depending on if their back spikes were extended. If their spikes stuck out, they would jump onto Mario, doing five damage; they would also be naturally spiked. If their spikes were retracted, they would simply Shell Toss for three damage instead. They would also sometimes retract their spikes in or out after being attacked.

Super Mario Advance 4: Super Mario Bros. 3
Bony Beetles also appear in the World-e level Swinging Bars of Doom from Super Mario Advance 4: Super Mario Bros. 3. They look and act the same exact way as in Super Mario World.

New Super Mario Bros. U
Bony Beetles are confirmed to appear in the upcoming game New Super Mario Bros. U, making their first 3D appearance. In behavior, they do exactly the same as in Super Mario World, where they walk around and suddenly stop to stick out their spikes.

Trivia

 * In Super Mario World, Bony Beetles have red feet, but the one showed in the credits cutscene for the game had blue feet.