The original Pokémon Red, Green, Blue, and Yellow games introduced 151 Pokémon species.
These creatures form Generation I of the Pokémon series, beginning with Bulbasaur and ending with Mew in National Pokédex order. The original games did not place every species in one cartridge: some Pokémon were exclusive to particular versions, so players needed to trade between games to complete the Pokédex.
The number 151 includes Mew, even though Mew was hidden and was not normally obtainable through standard gameplay in the original releases. This created a famous mystery around the final Pokédex entry and helped sustain interest in the games after launch.
Later generations expanded the total dramatically. Pokémon Gold and Silver added 100 more species, bringing the running total to 251. The number 151 therefore refers specifically to the first generation, not to the number of Pokémon in the franchise today.