Mew was secretly added to the original Pokémon Red and Green games to spark rumors.
Mew was not part of the publicly advertised 150 Pokémon that players could normally encounter. Satoshi Tajiri revealed that an extra Pokémon had been hidden in the games, creating speculation among players and encouraging continued interest after release.
Mew occupies National Pokédex number 151 and is classified as a Mythical Pokémon. Its hidden data was placed in a small amount of unused space that became available after debugging features were removed. Most players could not obtain Mew through ordinary gameplay, so official events and special distributions became important ways to receive it.
Mewtwo is a separate Pokémon created through genetic experimentation based on Mew. The two names and their story connection often cause confusion, but Mew was the secret addition that helped generate the original game's rumors. Mew later became central to the first Pokémon movie and several other stories.