Which generation of Pokémon introduced Mega Evolution?

Generation VI of Pokémon introduced Mega Evolution.

The mechanic debuted in Pokémon X and Y, which were released worldwide for the Nintendo 3DS on October 12, 2013. During battle, certain Pokémon can temporarily transform into a stronger Mega-Evolved form when their Trainer uses the required Mega Stone and Key Stone.

Mega Evolution differs from ordinary evolution because it lasts only during battle. A Pokémon returns to its normal form when the battle ends. The transformation can change a Pokémon’s appearance, abilities, statistics, and sometimes its type.

Generation VI also introduced the Kalos region, the Fairy type, Trainer customization, and the series’ first fully 3D mainline presentation. Later games expanded Mega Evolution with additional forms, although the mechanic was not used as the central battle transformation in every subsequent generation.