Missingno is a Pokemon from the first-generation Pokemon games that looks glitched and screwy. It appears when you abuse a glitch involving an old man and some islands to the south of Kanto. Missingno has the ability to warp reality around you when you encounter it, duplicating whatever item is in your sixth slot and wreaking havoc on your hall of fame data. He is also a joke character.
Though many believe it always looks like a block of black pixels, Missingno is capable of taking multiple forms that don't look so screwy, resembling Kabutops's fossil, Aerodactyl, or a ghost.