The glitched character found in Pokémon Red and Blue is called MissingNo.
MissingNo. is short for “Missing Number” and is not an official Pokémon species intended for normal encounters. It can appear when players exploit a programming glitch in the Game Boy versions of Pokémon Red and Blue.
The encounter became famous because it could display corrupted graphics and sometimes alter the quantity of an item in the player’s inventory. The glitch is connected to the game’s handling of wild Pokémon data and the player’s name during the Old Man demonstration in Viridian City.
MissingNo. is often described as a secret Pokémon, but that is inaccurate. It is an error-related placeholder produced by the game’s code, not one of the 151 officially designed Pokémon in the first-generation Pokédex.