The original Pokémon Red and Green games included 151 Pokémon species.
These creatures formed the first National Pokédex, numbered from Bulbasaur at 001 through Mew at 151. Players could not obtain every species in one version because Red and Green had version-exclusive Pokémon, so trading between games was part of the intended experience.
The first 150 species were generally encountered, battled, or evolved during the main adventure, while Mew was a Mythical Pokémon associated with special distribution events and later promotional methods. This helped make the phrase “catch ’em all” both a goal and a challenge.
The total is often confused with the number available in a particular game or with the 150 Pokémon originally known through ordinary gameplay. The franchise’s first generation is officially remembered as 151 because Mew belongs to the original Pokédex. Generation II later expanded the total to 251 with Pokémon Gold and Silver.