Mew was secretly added as a last-minute hidden creature to the original Pokémon games.
Mew was inserted into Pokémon Red and Green by programmer Shigeki Morimoto after the main development work was completed. Its data occupied unused space and was not intended to be openly revealed to players at launch.
Nintendo later promoted Mew through a special event connected with CoroCoro Comic. In 1996, twenty entrants were selected from the magazine promotion and had Mew uploaded to their game cartridges. The promotion helped create rumors and increased interest in Pokémon.
Mew is often confused with Mewtwo. Mewtwo is a separate Pokémon created from Mew’s genetic material and is obtainable in the original games through ordinary exploration. MissingNo. is a glitch, not an official species, while Celebi belongs to a later generation.