Which main-series Pokémon games introduced Pokémon breeding?

Pokémon Gold and Silver introduced Pokémon breeding to the main series.

Released for the Game Boy Color, Pokémon Gold and Silver debuted in Japan on November 21, 1999. They formed the second generation of the core Pokémon games and expanded the series with 100 new species.

Breeding takes place through the Pokémon Day Care. When compatible Pokémon are left there, they can produce an Egg, which hatches into a Pokémon. Ditto can breed with almost every Pokémon that is eligible for breeding, making it especially useful for producing Eggs.

Gold and Silver also introduced held items, a real-time day-and-night system, the Steel and Dark types, Shiny Pokémon, and the Johto region. Pokémon Crystal, the third game in the same generation, retained the breeding system but did not introduce it first. Later games expanded breeding with inherited moves, abilities, and regional forms.