The original first generation of Pokémon included 151 species.
These Pokémon debuted in the first-generation games, beginning with Bulbasaur at National Pokédex number 001 and ending with Mew at number 151. The collection included familiar early designs such as Pikachu, Charizard, and Blastoise.
Mew is an important reason people sometimes remember the total incorrectly. It was a hidden, Mythical Pokémon that was not normally obtainable through ordinary gameplay in the original games, but it was still counted as the 151st species. Mewtwo, by contrast, is number 150.
The first-generation total remained a defining part of Pokémon's early identity. Later generations added many more species, so the number of Pokémon in the franchise is now much larger than the original Pokédex. In anniversary discussions, however, 151 remains the landmark number associated with the series' 1996 beginning.