MissingNo. appears as Pokédex number 000 in Pokémon Red and Blue.
MissingNo. is an invalid or glitch Pokémon produced by manipulating the original games’ encounter data. The famous method involves viewing the Old Man’s catching tutorial in Viridian City, flying to Cinnabar Island, and surfing along the island’s eastern shore.
The glitch can create an encounter using leftover data that was not properly overwritten. Depending on the player’s name and the game version, the result may display different forms, including a scrambled block or an unused fossil-like sprite.
MissingNo. is not an intended species in the official first-generation Pokédex. Encounters can also alter item quantities and corrupt visual data, so the glitch is remembered as a technical curiosity rather than a normal way to complete the Pokédex.