The original 1996 Pokémon generation introduced 151 species.
Generation I debuted in Pocket Monsters Red, Green, and Blue, the Game Boy games that started the franchise. The species are numbered from Bulbasaur at National Pokédex number 0001 through Mew at number 0151.
The games’ story takes place in the Kanto region, where players receive a Pokédex and attempt to record Pokémon by catching, evolving, and trading them. Completing the original Pokédex required both game versions because some Pokémon were exclusive to one version or could only be obtained through trading.
Mew is the 151st species and was hidden in the original games as a secret Pokémon. Although it was not normally obtainable through standard gameplay, its existence became an important source of rumors and excitement. The original total is often confused with 150 because Mew was secret, but the completed Generation I count is 151.