Mew was secretly added to the original Pokémon games as the 151st species.
Mew was not part of the regular in-game progression and was hidden in the game data. Its existence helped fuel rumors among players, many of whom searched for ways to find a Pokémon beyond the visible 150 entries.
The creature was added late in development after debugging work freed enough space in the game's data. Satoshi Tajiri later revealed Mew publicly, and Nintendo used special events and distributions rather than ordinary gameplay to make it available to players.
Mew is distinct from Mewtwo, which is National Pokédex number 150 and appears as a powerful artificial Pokémon in the games' story. Mew's status as a Mythical Pokémon and its hidden origin made it one of the most important secrets in the franchise's early history.