The first generation of Pokémon games introduced 151 Pokémon.
These species appeared in the 1996 Game Boy games Pocket Monsters Red, Green, and Blue, followed by Pokémon Yellow. In National Pokédex order, Bulbasaur is number 0001 and Mew is number 0151.
The original games required trading between versions to obtain every species because some Pokémon were exclusive to particular versions. This version-exclusive design encouraged players to connect two Game Boy systems with a Game Link Cable.
A common mix-up is counting only the Pokémon available in one individual game, because no single original version contains every species without trading. The number 251 belongs to the combined total after Generation II added 100 more Pokémon.