The first generation of the Pokémon video-game series introduced 151 species.
These Pokémon debuted in the 1996 Game Boy games Pocket Monsters Red and Green in Japan. The international releases, Pokémon Red and Blue, used the same original group, although the Japanese Green version was not released overseas.
The National Pokédex numbers run from Bulbasaur at 0001 through Mew at 0151. This is why the original collection is often described as the “first 151.” Mew is included in the count even though it was difficult to obtain through ordinary gameplay and was distributed through special events.
The number is sometimes confused with the 150 species obtainable through normal story progression in the original games. Mew was the hidden final entry, while MissingNo. was a programming glitch rather than an official Pokémon species. Later generations expanded the Pokédex far beyond the original Kanto roster.