Pokémon Red and Green, the first Pokémon games, introduced 151 Pokémon species.
These original species make up what is now called Generation I, beginning with Bulbasaur and ending with Mew in National Pokédex order. The games’ central goal was to catch, train, and trade these creatures, making the number 151 a foundational fact in Pokémon history.
The number can cause confusion because Mew was difficult to obtain and was not normally encountered during the standard story. It was nevertheless part of the original 151 and occupied the final National Pokédex position in the first generation. MissingNo., a famous glitch, is not an official species and does not increase the count.
Later games added new generations, so the original total should not be confused with the current Pokédex count. Pokémon Gold and Silver added 100 more species, bringing the combined total to 251 and establishing the series’ continuing pattern of expanding its creature roster.