Which Pokémon game pair was the first to introduce the Johto region?

Pokémon Gold and Silver were the first games to introduce the Johto region.

Released for the Game Boy Color in Japan in 1999, the pair continued the story and mechanics of the original Pokémon games. Johto lies west of Kanto, and players can eventually return to Kanto after completing the main Johto adventure.

The games added 100 species, including the starter Pokémon Chikorita, Cyndaquil, and Totodile. They also introduced a real-time clock, day-and-night encounters, breeding, held items, and the Dark and Steel types.

Pokémon Crystal is an enhanced version of the same generation, but it was not the first Johto game pair. Ruby and Sapphire introduced Hoenn, Diamond and Pearl introduced Sinnoh, and Emerald was an expanded Hoenn title.