One of the new features in Pokémon X & Y is a new form of evolution: Mega Evolution. Unlike traditional evolution, Pokémon which have Mega Evolutions can only access these more powerful forms during battle and will revert back after the battle finishes. Mega Evolution is available after the player has bonded enough with their Pokémon and has also obtained the Mega Evolution stone for that particular Pokémon.
Mega Pokémon List
Listed in National Pokédex order. Type is noted only if it changes upon Mega Evolution. Ability noted in italics.
#003 Mega Venusaur (Defense & Sp. Defense up, Thick Fat)
#006 Mega Charizard Y (Sp. Attack up, Drought)
#006 Mega Charizard X (Fire/Dragon)
#009 Mega Blastoise (Mega Launcher)
#094 Mega Gengar (Shadow Tag)
#150 Mega Mewtwo X (Psychic/Fighting; Attack up, Steadfast)
#150 Mega Mewtwo Y (Sp. Attack up, Insomnia)
#181 Mega Ampharos (Electric/Dragon, Mold Breaker)
#248 Mega Tyranitar
#257 Mega Blaziken (Attack up, Speed Boost)
#303 Mega Mawile (Defense & Sp. Defense up, Huge Power)