The first generation introduced 151 Pokémon species in Pokémon Red, Green, and Blue.
Generation I began with the Japanese Game Boy releases Pocket Monsters Red and Green in 1996. International versions were released as Pokémon Red and Blue, and the same original group of species formed the first National Pokédex.
The sequence starts with Bulbasaur at number 001 and ends with Mew at number 151. Mew was not normally obtainable through ordinary gameplay in the original releases, which helped make it especially mysterious to early players.
The number 151 is often confused with the number of Pokémon available in a particular version. Because some species were exclusive to one version, completing the full collection required trading between games. Later generations added new species, while keeping the original 151 central to the franchise’s identity.