Marty the Thwomp is one of the non-playable characters in Lego Dimensions 2: The Rise of Enoch, from Nintendo's subfranchise Super Mario Bros.
Marty is a green Thwomp found behind jail bars in Mario Kart 64's Bowser's Castle. He is laughing and looks at you through the bars. He appears in the first room of the castle, the jail bars being in front of the first turn into the corridor into the next room. He returned in Mario Kart Wii, though he was no longer green.
The reasons for his imprisonment is unknown, at least officially.