Mew was hidden in the original Pokémon Red and Green games to spark rumors and sustain player interest. It was not normally obtainable through the standard adventure.
Mew was added late in development, after the team removed unused data and created a small amount of free space. Its secrecy encouraged players to exchange rumors about how to find the mysterious 151st Pokémon.
Nintendo later distributed Mew through special events rather than ordinary in-game encounters. Mewtwo is a separate Pokémon: it is numbered 150 and is obtainable near the end of the original games. MissingNo. is also different, representing a well-known glitch rather than an official species.