The Metal Parasite

The Metal Parasite is an alleged enemy found mostly in Hazy Maze Cave and Cavern of the Metal Cap. It takes the appearance of a glob of varying sizes with Metal Mario's textures.

Behavior
Supposedly, in certain personalized copies of Super Mario 64, the player can encounter The Metal Parasite in Hazy Maze Cave or Cavern of the Metal Cap. The size of the goopy parasite varies, some globs are small and tucked away in corners, while some are so large they cover up entire sections of the course, most notably in the Toxic Maze section of HMC. Once the player gets near, The Metal Parasite slowly inches towards the player in an attempt to touch them.

If the player were to come in contact with The Metal Parasite, it'll be absorbed by Mario as he plays his electrocuting animation, though much more corrupted. Afterwords, Mario will be in an enhanced Metal Mario state. As the infected Metal Mario, he is immune to all type of attacks, and cannot die, even by falling into pits, as he is simply launched out of it similar to that of the gust of wind found in Tall, Tall Mountain. It is said that this state is kept throughout the game, even after leaving Hazy Maze Cave or Cavern of the Metal Cap. The effects of the parasite are permanent throughout the playthrough, and the only known way to revert the effects is to erase the player's save file.

While under the effects of The Metal Parasite, the player gains the ability to phase through gates and glass, similar to that of the Vanish Cap ability, as well as walk up walls. The player's punches and kicks are also enhanced with extra range. Wherever the player moves, a trail of metal goop is left behind them, spreading The Metal Parasite even further, and by coming in contact with an enemy or NPC, they are said to turn all their textures metal, now being completely invincible, when touching water, the entire body of water is covered in The Metal Parasite, making it harder to see while underwater. All of the music in the game's instruments are also replaced with the instruments used in the Metal Mario theme.

Speculation
While the existence of The Metal Parasite is heavily debated on, some theories have sprouted up about why The Metal Parasite theoretically exists. A commonly spread theory is that The Metal Parasite is a personification of the Personalisation A.I, attempting to take control of the player's character in order to spread more of it across the game. It's relatively simplistic design also supports this theory, as it is simply a mess of polygons with Metal Mario's textures.

Another theory states that The Metal Parasite could've simply been a cut alternative to the Metal Cap. Due to how different the Metal Cap is in comparison to the Wing Cap and Vanish Cap, it's possible that instead of being a cap, it could've been a fluid Mario could touch to turn metal. Why it spreads around or why it stays on permanently is unknown, though it could be a mix of the A.I and/or bugs in the code.

Interestingly, the way in which the Metal Parasite is an enemy that gives Mario a powered-up form is very similar to the way that Wario uses power-ups in the Wario Land series. It's possible that the Metal Parasite is a remnant from Super Mario 64's earliest days of development as Wario Land 64, although this is merely circumstantial.

Trivia

 * When using the Metal Box item in Super Smash Bros. Ultimate, the animation of the fighter turning into their metal form has them covered in a liquid-like metal similar to that of the Metal Parasite. It's possibly that this idea could've been carried over from Super Mario 64 to Smash Bros. Ultimate, though this is unconfirmed.