Which games introduced Mega Evolution to the Pokémon video-game series?

Pokémon X and Y introduced Mega Evolution to the Pokémon video-game series.

Released worldwide for Nintendo 3DS in October 2013, Pokémon X and Y are the sixth-generation main-series games. Mega Evolution allowed certain fully evolved Pokémon to temporarily transform during battle by using a Mega Stone, usually while holding a strong bond with their Trainer.

The feature was distinct from ordinary evolution because it lasted only during battle and generally required a specific stone connected to a particular Pokémon. Examples introduced in these games included Mega Lucario, Mega Charizard X, Mega Charizard Y, and Mega Mewtwo X and Y.

Players sometimes associate Mega Evolution mainly with Pokémon Omega Ruby and Alpha Sapphire because those 2014 remakes added many more Mega forms. However, X and Y established the mechanic first. The same games also introduced the Fairy type and were the first main-series Pokémon titles presented with fully rendered 3D polygonal graphics.