Marty is a green Thwomp that is behind some jail bars in the N64 version of Bowser's Castle. He's 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 is a regular-colored Thwomp in Mario Kart Wii.
There are rumors that say you can unlock him in Mario Kart 64, which are not true.