Which Pokémon games first introduced the Mega Evolution feature?

Pokémon X and Y first introduced the Mega Evolution feature.

Released worldwide on October 12, 2013, Pokémon X and Y were the first main-series games to use Mega Evolution. The mechanic temporarily transforms certain fully evolved Pokémon during battle, usually through a compatible Mega Stone and a special trainer-held item.

Mega Evolution gave familiar Pokémon new forms, altered statistics, and sometimes different abilities or types. Examples include Mega Charizard X, Mega Charizard Y, and Mega Mewtwo Y. The feature became one of the defining innovations of Generation VI.

Mega Evolution is easy to confuse with later mechanics such as Z-Moves, Dynamax, and Terastallization. Those systems debuted in later generations and work differently. Pokémon Omega Ruby and Alpha Sapphire expanded the Mega Evolution roster, but they did not introduce the feature; X and Y did.