Metal Mario

Metal Mario is a powered-up version of Mario debuting in Super Mario 64. Upon wearing the Metal Cap, the heroic plumber will be coated entirely in metal. In this form, he is impervious to any type of damage and is able to walk underwater due to how heavy he is.

Since the power-up's debut, Metal Mario has reappeared as a seperate entity from Mario. First appearing in the Super Smash Bros. series starting with the original on the Nintendo 64, he acts as a silent mid-boss and is characterised like a statue. With his resurgence in Mario Kart 7, he's given a filtered voice done by Charles Martinet saying spins on Mario's own catchphrases.

Theorising
Despite never butting heads with the character he takes his form from, Metal Mario is referred to as Mario's rival in official descriptions of the character. It's strange how Metal Mario has become his own character despite never appearing as such in his original appearance, and it has led many to believe that he was originally intended to have a bigger role in Super Mario 64.

Metal Parasite

 * Main article: The Metal Parasite

A rather unknown anomaly associated with Metal Mario is an unused enemy affectionately called the Metal Parasite. This enemy takes the form of a metallic blob, sporting the same textures as Metal Mario and the Metal Cap. If Mario comes into contact with this blob, he will become Metal Mario albeit with complete invincibility and the ability to turn other enemies and characters metal as well. The existence of this anomaly is relatively unknown, but it is speculated to be an early version of the Metal Cap.

Trivia

 * The texture Metal Mario uses is the default metal texture of the SGI workstation. Oddly, Super Mario 64 is the only game this texture is used; in his future appearances, such as Super Smash Bros., he uses a different texture. Even with Metal Mario's return in Super Mario Odyssey using the game model from Super Mario 64, he still uses a different metallic texture. This could relate to the Silicon Graphics curse on the iceberg.
 * The metal texture is also used in The Legend of Zelda: Ocarina of Time.
 * Super Mario 3D All-Stars marks the texture's first reappearance since the original Super Mario 64 in the Mario series, however unlike other textures associated with Mario it isn't upscaled.