Mew was the hidden Pokémon added to Pokémon Red and Green after development was completed.
Mew was not part of the ordinary player-facing Pokémon roster when the games were originally designed. Its data was added late, occupying unused space in the game cartridge. Because players could not normally encounter it during the main adventure, rumors about the mysterious 151st Pokémon spread quickly.
The character was later promoted through a CoroCoro Comic lottery in Japan. Selected winners could send in their cartridges so Mew could be added, turning the hidden creature into a highly sought-after event Pokémon.
Mew is distinct from Mewtwo, which is encountered in the story after the Pokémon League. Mew is also classified as a Mythical Pokémon, while Mewtwo is generally classified as a Legendary Pokémon. Both are connected to the first-generation games but serve different roles.