Mew was the hidden Pokémon added to Pokémon Red and Green and helped fuel early rumors about the games.
Game Freak inserted Mew after the main development work was largely complete. Its data occupied a small amount of unused space, and the character was not part of the ordinary Pokédex-completion route in the original games.
Satoshi Tajiri later explained that revealing an extra Pokémon could encourage discussion, trading, and experimentation. Because Mew was not normally distributed through standard gameplay, players circulated rumors about secret methods and glitches that might reveal it.
Mew is distinct from Mewtwo, which is a major story encounter in the first games and is numbered 150 in the National Pokédex. Mew is number 151, making it the final entry in the original Pokédex and one of the franchise’s earliest mythical Pokémon.