Generation II introduced 100 new Pokémon species in Pokémon Gold and Silver.
The second generation expanded the original roster with creatures numbered 152 through 251 in the National Pokédex. The games were set mainly in the Johto region and were first released in Japan for the Game Boy Color on November 21, 1999.
Gold and Silver added more than new species. They introduced the Dark and Steel types, baby Pokémon, breeding, day-and-night changes, and differently colored Pokémon later known as Shiny Pokémon. The games also connected Johto to Kanto, allowing players to revisit the original region after completing the main adventure.
The number is sometimes confused with the total Pokédex size after Generation II. The first generation had 151 species, and adding 100 new species produced a total of 251 Pokémon in the second-generation National Pokédex.