Pokémon Gold and Silver added 100 Pokémon species to the series.
The second-generation games expanded the original Pokédex with species numbered from Chikorita through Celebi in National Pokédex order. This raised the total number of known Pokémon from 151 to 251. The new creatures came from the Johto region, although the games also allowed players to return to Kanto after completing the main Johto adventure.
Among the newly introduced species were the starters Chikorita, Cyndaquil, and Totodile; the legendary Pokémon Ho-Oh and Lugia; and baby Pokémon such as Pichu, Cleffa, and Igglybuff. The generation also added new evolutions for several earlier species, including Crobat, Steelix, and Scizor.
The number 100 refers to newly introduced species, not the total Pokédex size in the games. Because the first generation contained 151 Pokémon, the combined total became 251. Pokémon Crystal later expanded the second-generation experience but did not create a separate group of 100 additional species.