The original Pokémon games introduced 151 Pokémon species, from Bulbasaur at National Pokédex number 001 to Mew at number 151.
These creatures formed the first generation of Pokémon and populated the Kanto region. Players encountered most of them through normal gameplay, while others required trading between versions, special events, evolution, or unusual methods. The version-exclusive Pokémon encouraged players to connect with friends who owned the other game.
The number 151 includes Mew, even though Mew was not normally obtainable during the main adventure. Mew was added late in development and became famous because its existence was initially hidden from most players. That is why some discussions distinguish between the 150 Pokémon available through ordinary game progress and the full original total of 151.
Later generations expanded the Pokédex dramatically, but the original count remains a central milestone in Pokémon history. It is also why early marketing often used the slogan “Gotta Catch ’Em All” in connection with the first generation.