Which Pokémon game first introduced the ability to breed Pokémon?

Pokémon Gold and Silver first introduced the ability to breed Pokémon.

The second-generation games added the Pokémon Day Care, where compatible Pokémon could produce an Egg. Players could then hatch the Egg by carrying it while walking a required number of steps. This system expanded Pokémon from a collection of battle-ready creatures into a deeper system involving inheritance, moves, and team planning.

Breeding also introduced Pokémon Eggs as a major gameplay feature. In many cases, the species that hatches is based on the female Pokémon or the non-Ditto parent, while certain moves can be passed from a parent to its offspring. The games therefore made it possible to develop Pokémon with particular moves or characteristics rather than relying only on wild encounters.

Breeding is not the same as catching baby Pokémon in the wild. Some baby Pokémon, such as Pichu and Cleffa, were introduced in Gold and Silver and are obtained from Eggs. Later generations added more breeding rules, abilities, and items, but the basic system began in Johto.