Which Pokémon games introduced the Mega Evolution battle mechanic?

Pokémon X and Y introduced the Mega Evolution battle mechanic.

Released for the Nintendo 3DS in 2013, Pokémon X and Y added Mega Evolution as a temporary transformation available to certain Pokémon during battle. A Pokémon generally needed its corresponding Mega Stone, while the Trainer used a Key Stone to activate the transformation.

Mega Evolution could change a Pokémon’s appearance, statistics, type, or ability. It was distinct from ordinary evolution because it lasted only during battle and usually ended when the battle was over. Players could normally activate only one Mega Evolution per battle.

The mechanic became a major feature of Generation VI and returned in later games, spin-offs, and other Pokémon media. It is sometimes confused with Dynamax and Gigantamax, which debuted in Pokémon Sword and Shield, or Terastallization, introduced in Pokémon Scarlet and Violet.