Metal Mario

Metal Mario is a form that Mario can take in Super Mario 64. In the Super Smash Bros. series, Metal Mario is featured as a standalone character and one of Mario's forms, which he can temporarily transform into through the Metal Box, as with any of the fighters.

Background information
In Super Mario 64, Metal Mario can walk underwater, where he has unlimited air and cannot drown. He is also invulnerable to toxic gas, and can run into enemies to defeat them. In Super Mario 64 DS, the ability is instead used by Wario.

Super Smash Bros.
In Super Smash Bros., Metal Mario is always featured in the ninth stage of 1P Game, and is fought in his own unique stage, Meta Crystal. Metal Mario mainly walks back and forth on the stage, and rarely attacks. Because he is made of metal, Metal Mario has to usually take a lot of damage before he can be KO'd.

Super Smash Bros. Melee
In Super Smash Bros. Melee, Metal Mario is instead fought in the penultimate stage of Adventure, specifically the Battlefield stage. After the Fighting Wire Frames have been defeated, Metal Mario falls out of the sky. If Luigi has been unlocked, Metal Luigi teams up with Metal Mario.

Aside from this, Metal Mario is one of the possible opponents in Classic's metal stage. There is also a trophy of Metal Mario that can be acquired randomly.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, Metal Mario does not appear as a specific character, but it is possible for him to appear in the metal stage of Classic Mode. Just like in Super Smash Bros. Melee, Mario can temporarily transform into Metal Mario by using a Metal Box. In Special Brawl, Metal was introduced as one of the options. Activating it applies a metallic appearance to the Mario graphic featured on the options screen, just as any how any option changes his appearance accordingly.

Super Smash Bros. for Nintendo 3DS / Wii U
In Super Smash Bros. for Nintendo 3DS / Wii U, Metal Mario once again does not appear as a standalone character, but Mario can still use a Metal Box to temporarily assume this form. Metal returns as an option in Special Smash, allowing Mario to assume the form for an entire battle. Mario returns in the Special Smash rules menu, this time in the form of a model instead of a graphic, and activating any option, including Metal, applies that property to his model.

Super Smash Bros. Ultimate
In Super Smash Bros. Ultimate, Metal Mario is featured as the final boss in Bowser's Classic Mode route.