Which Pokémon games introduced the second generation and 100 new Pokémon species?

Pokémon Gold and Silver introduced the second generation and 100 new Pokémon species.

The games were released in Japan in 1999 for the Game Boy Color. Their story takes place in the Johto region, where players encounter new species alongside many Pokémon from the original games. Gold and Silver expanded the series with features including a day-and-night cycle, breeding, held items, Pokémon eggs, and the Dark and Steel types.

The second generation also connected directly to the first. After completing the main Johto adventure, players can travel to Kanto and challenge its Gyms. This return trip is one reason the games are sometimes remembered as containing two full regions, although Johto is their primary setting and the game’s 100 new species are numbered from Chikorita through Celebi.