Marty the Thwomp

The name "Marty" is not official, and is instead a name created by the players of Mario Kart 64.

Marty the Thwomp is a Thwomp found in the Bowser's Castle track of Mario Kart 64. They were a green Thwomp (though the green coloration simply came from the lighting) locked behind bars within the castle, simply serving as a background detail with no bearing on gameplay. Many believed that they could be released from their prison, and were even an unlockable character similarly to Luigi in Super Mario 64, despite all of these rumors seemingly being false. However, according to those who have used Stop N' Swop to personalized Mario Kart 64, Marty the Thwomp has actually become a playable character.

In-Game
As previously stated, Marty was seen in the Bowser's Castle track of Mario Kart 64, simply being a background character at the end of a hallway locked behind bars. Due to the lighting of the room, they appear green, but are actually the same color of the other Thwomps in the level. Upon entering the castle, laughing can be heard, which is believed to come from Marty. Within the prison cell, they appear to be smiling. They cannot be interacted with, and do not interact with the player.

Anomalous Behavior
According to many, you can somehow free Marty from his prison. This usually either requires driving around in a specific manner in the track to somehow open up the cage with a specific character, most often Bowser, finding a key somewhere in the track or even another area, or running into the bars at maximum speed with a heavyweight racer to destroy them. Upon releasing them, they can give the player a random item, unlock a hidden passage to a hidden track or to another part of the stage, or even unlock a new racer, most commonly Marty himself. As one would expect, they are a heavyweight racer, but otherwise play normally. They usually replace another racer in the game, most commonly Donkey Kong, potentially referencing how Donkey Kong took the place of the unused Magikoopa racer, though they can reportedly appear in a 9th slot or even a second page on the player select.

Theorizing
It is believed that most reports of Marty the Thwomp being playable were made up by players like many other playground rumors, as Thwomps have never been playable in the Mario Kart series, and this specific Thwomp was only a background detail and no other unlockable characters exist within the game. Why the A.I. would choose to make them playable is also not understood, though it may have potentially been based off of an actual unused unlockable character mechanism within earlier builds of the game. However, freeing Marty and obtaining some other reward is potentially more likely as an unused mechanism as an easter egg, excluding the methods of running around the track in a specific pattern with a specific character which is almost definitely exclusively playground rumors. Why it would have been removed is unknown if it was created by the developers, though it may have been deemed "too obscure" or unfair for other players unaware of it playing the track.

Trivia

 * In the remake of the Bowser's Castle track seen in Mario Kart Wii, Marty returns in the background, though they do not remain green.