The original Pokémon games introduced 151 Pokémon species.
Pokémon Red and Green established the first-generation Pokédex, beginning with Bulbasaur and ending with Mew. These 151 species included the creatures available through normal gameplay, version-exclusive encounters, evolutions, trades, and special events.
The figure 151 is important because the games’ central collection goal was to obtain every species in the original Pokédex. Players could not complete the collection using only one version: Red and Green, and later Red and Blue internationally, contained different Pokémon that had to be traded between games.
Some people remember the original total as 150 because Mew was hidden and not normally obtainable through standard gameplay. Mew was nevertheless counted as the 151st Pokémon. The number later rose to 251 when the second generation introduced 100 additional species in Pokémon Gold and Silver.