The first-generation Game Boy games Pokémon Red, Green, and Blue introduced 151 Pokémon species.
These creatures form the original National Pokédex, numbered from Bulbasaur at 0001 through Mew at 0151. The Japanese versions were Pocket Monsters Red, Green, and Blue, while the international releases used the names Pokémon Red and Blue. Pokémon Yellow later joined the same generation.
The number 151 includes Mew, even though Mew was not normally obtainable through ordinary gameplay in the original releases. Mew was a secret, late addition used in promotional events, making it easy for early players to think the first Pokédex contained only 150 Pokémon.
The first generation was followed by Pokémon Gold and Silver, which added 100 more species. That expansion brought the total to 251 and introduced the Johto region. Later generations continued to increase the National Pokédex, but 151 remains the defining number of the franchise’s original era.