Which Pokémon game first introduced Pokémon breeding?

Pokémon Gold and Silver first introduced Pokémon breeding.

Released in Japan in 1999 for Game Boy Color, Gold and Silver added a daycare system where compatible Pokémon could produce Eggs. Players could then hatch those Eggs, creating a new way to obtain Pokémon and pass certain traits or moves between generations. The feature expanded the series beyond catching wild creatures and trading with friends.

Breeding is often associated with Ditto, because Ditto can breed with many Pokémon species, but Ditto was already present in the first-generation games. Gold and Silver also introduced Pokémon Eggs through the story and added new baby Pokémon, including Pichu, Cleffa, and Igglybuff. Pokémon Crystal, released later, used the same breeding system rather than introducing it.