Mew was added to the original Pokémon games as an internal prank.
According to the game’s development history, programmer Shigeki Morimoto inserted Mew after the final debugging work. The creature was not intended to be openly revealed to players and was not part of the ordinary collection path at launch.
Mew’s existence became a source of rumors and mystery. Nintendo later distributed it through promotional events, turning the hidden character into an official part of the franchise’s mythology.
Mew is distinct from Mewtwo. Mew is the original Mythical Pokémon associated with hidden data and genetic ancestry, while Mewtwo is a genetically engineered Pokémon created from Mew’s DNA and serves as the central figure in the first Pokémon movie.