Mew was the hidden Pokémon added to the original Pokémon games after development.
Mew was placed in Pokémon Red and Green late in development, after the debugging process had created a small amount of extra space in the game data. The creature was not intended to be found through ordinary gameplay, which helped generate rumors and mystery around its existence.
Nintendo and Game Freak later distributed Mew through special events and promotions. Because it was numbered 151, Mew completed the original National Pokédex after Mewtwo at number 150.
Mew is often confused with Mewtwo, but they have different roles. Mew is a mythical Pokémon associated with ancient genetic origins, while Mewtwo is a laboratory-created Legendary Pokémon. MissingNo. is another famous hidden or glitch-related phenomenon, but it is not an official species.