Which Pokémon generation introduced 100 new species in Gold and Silver?

Generation II introduced 100 new Pokémon species in Pokémon Gold and Silver.

Released in Japan in 1999 for the Game Boy Color, Gold and Silver expanded the original roster from 151 to 251. The new species belonged mainly to the Johto region, which was connected to the earlier Kanto setting through the games' geography and post-game content.

Gold and Silver also added several systems that became important parts of Pokémon. They introduced Pokémon breeding, held items, day-and-night time cycles, genders for most Pokémon, and two new types: Dark and Steel. The games also allowed players to revisit Kanto after completing the Johto storyline.

Generation II is sometimes confused with the second Pokédex total, 251. The generation itself added 100 species; 251 is the cumulative number after combining those 100 with Generation I's original 151.