Skeleton Mario

Skeleton Mario is an original form that appears in Super Mario-kun, mostly in the Super Mario World arc.

The first occasion in which this form is shown is the last chapter of volume 1, as one of the hypothetical future transformation of the Mario series. Mario can transform into a skeleton for 3 minutes after collecting a Hone, but has no known advantages or powers.

In volume 2, a Fishbone attacks the heroes by shooting a "Bone Ray", turning Luigi and Yoshi into skeletons. Before Mario can be hit, though, a Super Star grants him invincibility, and he defeats the enemy. Meanwhile, Luigi and Yoshi ask how they will turn back to normal.

In volume 3 and 6, to withstand the heat, Yoshi removes his skin like a shirt, revealing his skeleton (without the need of any power-ups or attacks), creeping out the Mario Bros.

Trivia

 * A skeletal Mario would appear in the games in Super Mario Galaxy as a death animation from electric attacks, and as a costume in Super Mario Odyssey.