The original Pokémon games' Pokédex included 151 species.
Pokémon Red and Green introduced the first generation of Pokémon, numbered from Bulbasaur at 001 through Mew at 151. The games’ central goal was to encounter and record these creatures, although completing the full Pokédex required trading between versions because some Pokémon were exclusive to one game.
The first 150 Pokémon were obtainable through ordinary game data and version-specific encounters, while Mew was a secret Mythical Pokémon added as a surprise. Mew’s existence was not broadly publicized at launch, helping create rumors and renewed interest in the games.
The number 151 is sometimes confused with 150 because Mewtwo is the final commonly encountered Pokémon in the main story and early promotional material emphasized the original 150. However, the National Pokédex sequence places Mew immediately after Mewtwo, making 151 the complete count associated with the original generation.