Mew was secretly added to the original Pokémon games as National Pokédex number 151.
Mew was not part of the ordinary capture sequence in Pokémon Red and Green. Game Freak programmer Shigeki Morimoto added it after the game’s debugging process, using a small amount of space that became available when development data was removed. The developers kept Mew secret from the public at first.
Mew’s hidden status generated rumors among players and helped sustain interest in the games. Nintendo later distributed it through special events and promotions, rather than through the normal story or standard wild encounters. This made Mew one of the earliest examples of an event Pokémon.
Mew is distinct from Mewtwo, which is number 150 and is encountered through the game’s narrative and post-game content. Mew is classified as a Psychic-type Mythical Pokémon and is described as having genetic connections to many other Pokémon. Its National Pokédex placement makes it the final member of the original 151.