He's adorable wherever you build him. Puzzle piece cutie mark 'cause he's good at solving puzzles/mysteries. ;) (Pinkie should ask him or Data along on her next train trip.) That's actually quite a perfect cutie mark for him, because he's a sort of puzzle himself...doesn't quite fit in, has so much mystery surrounding himself, wants to solve it and find the other puzzle pieces that will fit his, and complete his picture. He needs to find his missing piece, and the puzzle that requires him in order to be completed. :*} ALSO, a puzzle piece is, like, an irregular shape...and he likes to take on various shapes, 'cause he's a Shapeshifter...his relaxed gelatinous state is certainly irregular. ;) He can, like a liquid, take the shape of a container in order to fit. In effect, he can alter and mold himself to suit his surroundings and companions--at least, to a degree. Balloon background, because Pinkie throws EVERYPONY parties and wants to be their instant best friend ("instant chummy," as Lucy Ricardo would say), no matter how endearingly awkward and nervous they are.