The original Pokémon Red and Green games featured 151 Pokémon species to collect.
That total included the first-generation Pokémon, from Bulbasaur through Mew. Completing the Pokédex was one of the games’ central goals, but no single Japanese cartridge contained every species. Players had to trade with another version because Red and Green distributed different Pokémon.
The number 151 includes Mew, even though the Mythical Pokémon was not normally obtained through standard gameplay. Mew was hidden in the games and later distributed through special events, helping create rumors and sustained interest among players.
The total is often confused with 150 because Mew was exceptionally difficult to obtain. It is also sometimes confused with 151 as a count of creatures available in one version, which is not correct: version-exclusive Pokémon and trading were part of the original design. Later generations expanded the Pokédex substantially beyond the first 151 species.