The original Generation I Pokémon games introduced 151 Pokémon species.
Pokémon Red, Green, and Blue established the core ideas that define the series: catching Pokémon, training them, battling, trading, and completing a Pokédex. The games were released for the Game Boy, beginning with Pocket Monsters Red and Green in Japan.
The 151 species run from Bulbasaur, National Pokédex number 001, to Mew, number 151. Mew was difficult to obtain and was not normally available through ordinary gameplay, but it was still counted as part of the original Pokédex. Mewtwo, number 150, preceded it in the numbering.
A common mix-up is confusing the original 151 with the total number of Pokémon added by later generations. Generation II added 100 more, bringing the running total to 251. The first-generation figure remains central to Pokémon's early history and its 30th-anniversary legacy.