Generation I introduced 151 Pokémon species to the main Pokémon series.
These species first appeared across Pokémon Red, Green, Blue and Yellow, beginning with Bulbasaur at National Pokédex number 001 and ending with Mew at number 151. The group is commonly associated with the Kanto region, the setting of the original games.
The original 151 include familiar Pokémon such as Pikachu, Charizard, Gengar and Dragonite. They also established the three-stage evolution lines, type matchups and legendary Pokémon that became defining parts of the series. Mew was unusual because it was not normally obtainable through standard gameplay in the original releases.
Later generations added new regional Pokédexes and expanded the National Pokédex. Generation II added 100 species, while later generations varied in size. The phrase “the original 151” therefore refers specifically to the first-generation species, not to every Pokémon available in modern games.