Mew was secretly added to the original Pokémon games as an internal development prank.
Programmer Shigeki Morimoto inserted Mew after the games were nearly complete. The team had created a small amount of unused space by removing debugging material, allowing the secret Pokémon to fit into the final cartridges without being part of the ordinary player-facing design.
Mew was not initially intended to be obtainable through normal gameplay. Its existence became a subject of rumors, and Nintendo later distributed it through promotional events. The mystery helped sustain interest in the games and encouraged players to compare discoveries.
Mew is National Pokédex number 0151 and is classified as a Mythical Pokémon. It is often confused with Mewtwo, which is number 0150 and has a prominent role in the first Pokémon movie. Although the names are related, Mew and Mewtwo are distinct species with different origins and stories.