Which two Pokémon games introduced the second generation and the Johto region?

Pokémon Gold and Silver introduced the second generation and the Johto region.

The games were first released in Japan for the Game Boy Color in 1999. They added 100 new Pokémon, bringing the series total to 251. Their story takes place mainly in Johto, a region connected to the earlier Kanto setting.

Gold and Silver expanded the formula with a real-time day-and-night system, Pokémon breeding, held items, two new types, and a redesigned interface. The games also allowed players to return to Kanto after completing the Johto adventure, making the earlier region part of the postgame experience.

The two versions had different version-exclusive Pokémon, continuing the trading tradition of the first generation. Pokémon Crystal followed as an enhanced version. The pair is sometimes confused with Ruby and Sapphire, which began the third generation and introduced Hoenn, or with HeartGold and SoulSilver, later Nintendo DS remakes of the Johto games.