Mew was secretly added as an internal prank to the first Pokémon games.
Developer Shigeki Morimoto inserted Mew after the game’s debugging work was complete. The tiny amount of free space created by removing debugging data made it possible to include the hidden Pokémon without changing the finished game’s main content.
Mew was not intended to be openly obtainable through ordinary gameplay. Its existence was initially unknown to most players, but rumors spread after Nintendo later distributed it through promotional events. This mystery helped sustain interest in the games and contributed to the culture of searching for hidden Pokémon secrets.
Mew is often confused with Mewtwo because the names are related and both are associated with the first generation. Mewtwo is a separate Pokémon created through genetic experimentation in the story. Mew is listed as National Pokédex number 151, while Mewtwo is number 150.