Mew was the Pokémon secretly hidden in the original Pokémon Red and Green games.
Mew was not part of the publicly announced 150-Pokémon roster. Satoshi Tajiri revealed the extra Pokémon after development, and its hidden presence helped sustain curiosity and discussion around the games.
Mew occupies National Pokédex number 151 and is classified as a Psychic-type Mythical Pokémon. It was not normally obtainable through standard play in the original releases, which made it especially mysterious to early players.
Mewtwo is a separate Pokémon numbered 150, despite its name suggesting a connection. Celebi belongs to the second generation, while MissingNo. is a well-known glitch rather than an official species.