Mew was secretly added late to Pokémon Red and Green.
Mew was inserted after development was effectively complete, using a small amount of unused programming space. Its existence was not widely known at launch, and Nintendo did not initially publicize it as part of the ordinary game content.
Because Mew was not normally obtainable through the standard story, rumors spread among players. Those rumors encouraged experimentation, trading, and discussion, helping keep interest in the games alive. Later, Nintendo distributed Mew through special events and promotions.
Mew is National Pokédex number 151, while Mewtwo is number 150. The two are related in the story, but Mew is the secret late addition. Mew’s unusual status also explains why early players sometimes counted only 150 obtainable Pokémon before special distributions became available.