Which Pokémon games introduced Pokémon breeding as a regular game feature?

Pokémon Gold and Silver introduced Pokémon breeding as a regular game feature.

Released for the Game Boy Color, Gold and Silver added the Johto region and expanded the series beyond the original 151 species. Their Pokémon Daycare system allowed compatible Pokémon to produce an Egg, which could later hatch into a young Pokémon.

Breeding added a new layer to team building because offspring could inherit characteristics such as species-linked moves. The games also introduced the Egg item and established rules involving breeding groups, gender, and Pokémon that cannot breed.

Players sometimes credit later games with breeding because later generations expanded the system substantially. However, its regular inclusion in the main series began with Gold and Silver. Ditto became especially useful because it can breed with many Pokémon, while Legendary and Mythical Pokémon generally belong to the No Eggs Discovered group.