Marty the Thwomp

Marty the Thwomp is a fan characterization of a Thwomp found behind bars in the Mario Kart 64 interpretation of Bowser's Castle. Combined with its apparent imprisonment, its unique green appearance and the fact that it laughs at nearby racers made "Marty" the subject of various rumors, in a similar vein to Luigi's alleged presence in Super Mario 64.

The most notable rumors claimed that Marty could be unlocked as a playable character, using hacks and doctored footage as evidence. A popular variation claims that playing as Bowser on Bowser's Castle, circling the garden statue three times, driving a lap backwards, and circling the statue three more times in the opposite direction will release Marty from the cage, playing a 1-Up sound effect.

Disconfirmation
Analysis of Mario Kart 64 's ROM has failed to produce evidence of any unlockable characters. Moreover, there are no assets for green Thwomps in the game's files—"Marty" only appears green due to the yellow lighting in the cage. (This lighting was changed in Mario Kart Wii.) Ergo, the Thwomp would lose its signature green appearance if taken away from its intended position.

One of the original videos circulating the Marty rumor was uploaded on April Fools' Day.

Based on this, it is likely that the caged Thwomp was meant as a simple setpiece for the race course, and some players had read too far into it.

The Mario Kart Racing Wiki suggests that the name "Marty" was chosen for the ease of image editing, due to sharing letters with real playable characters (especially the "MAR" of Mario).