MissingNo. is the glitch Pokémon-like character found in the original Pokémon games.
The name is short for “Missing Number,” referring to an invalid or unused Pokémon entry. In Pokémon Red, Green, and Blue, players could encounter MissingNo. by following a particular sequence involving the Old Man in Viridian City and then traveling to Cinnabar Island. Its appearance and effects depended on the game version and the player’s name data.
Encountering MissingNo. could corrupt visual data or cause unusual effects, including multiplying items in the sixth inventory slot. Because the games were not designed to present the character as a normal species, its behavior was inconsistent and sometimes disruptive.
MissingNo. is not one of the 151 official Generation I Pokémon. It is a programming artifact produced by the games’ handling of unused data. Its discovery became one of the most famous Pokémon glitches, helping fuel early internet rumors and discussions about hidden creatures.