Generation II added 100 new Pokémon species to the Pokémon franchise.
The generation began with Pokémon Gold and Silver, followed by Pokémon Crystal. Its new species run from Chikorita to Celebi and raised the total number of Pokémon from the original generation’s 151 to 251.
Generation II also introduced the Johto region, a day-and-night system, Pokémon breeding, held items, and two new types: Dark and Steel. After completing much of the Johto adventure, players can return to Kanto, making Gold, Silver, and Crystal unusual among the main games for containing two connected regions.
The figure 100 refers to species new to the generation, not the total Pokédex count in those games. That distinction explains why references may say Generation II has 100 new Pokémon but 251 Pokémon overall. The generation’s additions include popular species such as Lugia, Ho-Oh, Espeon, Umbreon, and Tyranitar.