Generation II added 100 new Pokémon to the series.
Generation II began with Pokémon Gold and Silver and later included Pokémon Crystal. These games introduced the Johto region while also allowing players to revisit Kanto after completing the main Johto adventure.
The new species included recognizable Pokémon such as Chikorita, Cyndaquil, and Totodile, along with baby Pokémon, evolutionary relatives of Generation I species, and legendary Pokémon such as Lugia and Ho-Oh. The additions expanded the series from 151 species to a total of 251.
Generation II also added important gameplay features, including breeding, held items, a day-and-night cycle, Pokémon genders, and shiny Pokémon. These systems made the new games feel like a substantial sequel rather than a simple roster update.
The figure 100 refers only to species newly introduced in Generation II. The games still included many Generation I Pokémon, so the complete Pokédex available across the generation contained 251 species.