Mew was the hidden Pokémon secretly added to the original Pokémon games.
Mew was inserted into Pokémon Red and Green near the end of development, after the game’s debugging work had freed a small amount of space. It was not intended to be part of ordinary gameplay and was not initially announced to players.
Rumors about the mysterious 151st Pokémon helped sustain interest in the games. Nintendo later distributed Mew through special promotional events, making it one of the earliest examples of an event-exclusive Pokémon. Its existence also helped reinforce the idea that the games contained secrets beyond the standard adventure.
Mew is often confused with Mewtwo, but the two are separate Pokémon. Mew is the Mythical Pokémon numbered 151, while Mewtwo is a genetically engineered Legendary Pokémon associated with the first movie and the endgame of the original games. The hidden creature in the launch titles was Mew.