Which Pokémon games first introduced Mega Evolution to the main series?

Pokémon X and Y first introduced Mega Evolution to the main Pokémon series.

Released for Nintendo 3DS in 2013, Pokémon X and Y added a temporary battle transformation called Mega Evolution. During a battle, certain Pokémon can transform when they hold the appropriate Mega Stone and their Trainer possesses a Key Stone. The transformation generally changes appearance, statistics, abilities, or type.

The mechanic differs from ordinary evolution because it ends when the battle ends. In most games that support it, only one Pokémon on a team can Mega Evolve during a battle. The feature gave older Pokémon new competitive roles and made familiar species central to the sixth-generation battle system.

Mega Evolution should not be confused with later mechanics such as Z-Moves, Dynamax, and Terastalization. Those systems were introduced in different generations. Mega Evolution also appeared in later remakes and spin-offs, but Pokémon X and Y were its original main-series debut.