In Mario Kart 64's Bowser's Castle, one can see what appears to be a green Thwomp locked up in a jail cell - this Thwomp is not actually green, but simply lit in a way that makes him appear as such.

This Thwomp would inspire urban legends claiming that players could bail him out through varying elaborate tricks, after which he would be playable under the name "Marty" (likely chosen for the ease of editing hoaxed images with the names Mario, Toad, and Yoshi)

When N64 Bowser's Castle returned in Mario Kart Wii, "Marty" would be made the color of the rest of the Thwomps, suggesting that his green complexion was simply an error and not an intentional design choice.