Which Pokémon games first introduced Pokémon breeding as a gameplay feature?

Pokémon Gold and Silver first introduced Pokémon breeding as a gameplay feature.

Released in Japan in 1999, the games added a Pokémon Day Care system in which compatible Pokémon could produce an Egg. The Egg could then hatch into a young Pokémon, creating a new way to obtain and raise creatures beyond catching them in the wild.

Breeding is governed by compatibility rules. Pokémon are assigned to one or two breeding groups, and compatible Pokémon left at the Day Care may produce an Egg. The offspring generally belongs to the mother’s species, while moves and other inherited characteristics can come from the parents. Ditto provides a special exception because it can breed with most Pokémon outside the No Eggs Discovered group.

Gold and Silver also introduced the real-time clock, held items, the Dark and Steel types, and Shiny Pokémon. Breeding therefore arrived as part of a much broader expansion of the series’ training and collection systems.