Which Pokémon game introduced Mega Evolution?

Pokémon X and Y introduced Mega Evolution. Released worldwide for the Nintendo 3DS in October 2013, the paired games launched Generation VI and added a temporary battle transformation that allowed certain fully evolved Pokémon to become stronger Mega forms.

Mega Evolution differs from ordinary evolution because it occurs only during battle and normally requires two linked items: a Mega Stone held by the Pokémon and a Key Stone carried by its Trainer. A Pokémon generally reverts to its normal form when the battle ends. The mechanic emphasized the games’ themes of bonds, evolution, and the relationship between Trainer and Pokémon.

Mega Evolution is often confused with later mechanics such as Z-Moves, Dynamax, Gigantamax, and Terastallization. Those systems were introduced in later generations and work differently. Pokémon X and Y also introduced the Fairy type, character customization, the Kalos region, and fully polygonal 3D presentation. Mega Evolution later appeared in Pokémon Omega Ruby and Alpha Sapphire, but those games expanded a feature that X and Y had already introduced.