Generation II introduced 100 new Pokémon species.
The second generation began with Pokémon Gold and Silver in Japan in 1999 and expanded the world beyond the original Kanto setting. Its new region was Johto, although players could later return to Kanto after completing the Johto portion of the adventure.
The 100 additions brought the Pokédex total from 151 to 251. Chikorita was the first new species in National Pokédex order, while Celebi was the last. The games also added Pokémon breeding, a day-and-night system, two new types—Dark and Steel—and version-specific mechanics that deepened the role-playing experience.
Generation II is sometimes confused with the total number of Pokémon available in Gold and Silver. The correct figure is the number of new species introduced by that generation, not the total Pokédex size. The generation’s 100 additions therefore represent a rise from 151 to 251 overall.