Mew was the secret Pokémon added late to the original Red and Green games.
Mew was inserted after the main development work had effectively been completed. Shigeki Morimoto added it as an internal surprise, and its data occupied a small amount of space left after other development material was removed. It was not intended to be openly revealed to players at launch.
The Pokémon became the subject of rumors because it was not normally obtainable through the standard story or Pokédex-completion route. Nintendo later distributed Mew through promotional events, turning the hidden creature into a major part of Pokémon’s early mythology.
Mew is National Pokédex number 151, while Mewtwo is number 150. The two names are often confused because Mewtwo was prominent in the first movie, whereas Mew’s original fame came from its secret status and event distributions.