The original Pokémon generation contained 151 species.
These creatures made up Generation I, introduced in the 1996 Game Boy games Pocket Monsters Red, Green and Blue. Their National Pokédex numbers run from Bulbasaur at 001 to Mew at 151.
The total can be confusing because the first Japanese releases did not reveal every Pokémon through ordinary gameplay. Mew was hidden in the games and became famous as a secret Mythical Pokémon, but it was still included in the original 151-species count.
Generation I is also sometimes described differently by region. Japan’s first pair was Red and Green, whereas international releases were called Red and Blue. Pokémon Yellow later revisited the same generation with Pikachu as the player’s companion. Later generations expanded the roster substantially, but the original 151 remain the foundation of the franchise’s early Pokédex.