Pokémon X and Y introduced Mega Evolution in 2013.
The games were released worldwide on October 12, 2013, for Nintendo 3DS. Mega Evolution allowed certain Pokémon to transform temporarily during battle when specific conditions were met, usually involving a compatible Mega Stone and a matching Key Stone.
Unlike ordinary evolution, Mega Evolution generally lasted only for the battle. It could change a Pokémon’s appearance, ability, statistics, and sometimes its type. Examples included Mega Charizard X, Mega Charizard Y, Mega Mewtwo X, and Mega Mewtwo Y.
The mechanic was not the same as ordinary evolution and was not available to every species. Pokémon X and Y also marked the main series’ first worldwide simultaneous release. Mega Evolution later appeared in other games, including Omega Ruby and Alpha Sapphire, but its debut belongs to the sixth-generation X and Y titles.