Pokémon X and Y/Mega Evolutions

Mega Evolution is a new mechanic introduced in Pokémon X and Y. Unlike normal evolution, Mega Evolution is only temporary, and requires a corresponding Mega Stone to be held by the Pokémon. Mega Evolutions will always increase a Pokémon's base stat total by 100, even reducing certain stats to compensate, and in some cases give Pokémon new abilities. A Pokémon only needs to Mega Evolve once each battle, after which the evolution will last the whole battle. Only one Pokémon can Mega Evolve in any battle, and their Mega Stones cannot be removed in any way, such as through Trick or Knock Off.

With the release of Omega Ruby and Alpha Sapphire, a similar mechanic called Primal Reversion is introduced. Like Mega Evolution, Primal Reversion requires the Pokémon to hold the correct item, which cannot be removed by moves like Knock Off. However, Primal Reversion is triggered automatically when the Pokémon enters the battle, and there is also no restriction on the number of times Primal Reversion can be used in a single battle.

Venusaur

 * Venusaurite Location: Given by Professor Sycamore if you chose Bulbasaur as a starter. Can be bought from old man in Lumiose City's Stone Emporium.

Charizard

 * Charizardite X/Y Location: Given by Professor Sycamore if you chose Charmander as a starter. Can be bought from old man in Lumiose City's Stone Emporium.

Blastoise

 * Blastoisinite Location: Given by Professor Sycamore if you chose Squirtle as a starter. Can be bought from old man in Lumiose City's Stone Emporium.

Beedrill
Introduced in Omega Ruby and Alpha Sapphire.

Pidgeot
Introduced in Omega Ruby and Alpha Sapphire.

Alakazam

 * Alakazite Location: In middle of room at lowest floor of Reflecting Cave (post-game only).

Slowbro
Introduced in Omega Ruby and Alpha Sapphire.

Gengar

 * Gengarite Location: Given by girl in Laverre City.

Kangaskhan

 * Kangaskhanite Location: In a dead end of Glittering Cave (post-game only).

Pinsir

 * Pinsirite Location: Pokémon X only, near the entrance of Santalune Forest (post-game only).

Gyarados

 * Gyaradosite Location: Around the waterfalls of Couriway Town (post-game only).

Aerodactyl

 * Aerodactylite Location: Given by person in Ambrette Town.

Mewtwo

 * Mewtwonite X/Y Location: Obtained after capturing Mewtwo.

Ampharos

 * Ampharosite Location: Given by person in Azure Bay.

Steelix
Introduced in Omega Ruby and Alpha Sapphire.

Scizor

 * Scizorite Location: Behind where Abomasnow is standing in the Frozen Cavern (post-game only).

Heracross

 * Heracronite Location: Pokémon Y only, near the entrance Santalune Forest (post-game only).

Houndoom

 * Houndoominite Location: Pokémon Y only, near some skaters at Route 16 (post-game only).

Tyranitar

 * Tyranitarite Location: Pokémon X only, at a dead end in Cyllage Gym (post-game only).

Sceptile
Introduced in Omega Ruby and Alpha Sapphire.

Blaziken

 * Blazikenite Location: Event only.

Swampert
Introduced in Omega Ruby and Alpha Sapphire.

Gardevoir

 * Gardevoirite Location: Held by Ralts traded from Champion.

Sableye
Introduced in Omega Ruby and Alpha Sapphire.

Mawile

 * Mawilite Location: On second floor of Shabboneau Castle (post-game only).

Aggron

 * Aggronite Location: Pokémon Y only, in a dead end in Cyllage Gym (post-game only).

Medicham

 * Medichamite Location: Near the lake in Laverre City (post-game only).

Manectric

 * Manectite Location: Pokémon X only, near the skaters in Route 16 (post-game only).

Sharpedo
Introduced in Omega Ruby and Alpha Sapphire.

Camerupt
Introduced in Omega Ruby and Alpha Sapphire.

Altaria
Introduced in Omega Ruby and Alpha Sapphire.

Banette

 * Banettite Location: In the Cave of Emptiness (post-game only).

Absol

 * Absolite Location: Given by rival after rival battle in Kiloude City.

Glalie
Introduced in Omega Ruby and Alpha Sapphire.

Salamence
Introduced in Omega Ruby and Alpha Sapphire.

Metagross
Introduced in Omega Ruby and Alpha Sapphire.

Latias
Introduced in Omega Ruby and Alpha Sapphire.

Latios
Introduced in Omega Ruby and Alpha Sapphire.

Kyogre
Introduced in Omega Ruby and Alpha Sapphire.

Groudon
Introduced in Omega Ruby and Alpha Sapphire.

Rayquaza
Introduced in Omega Ruby and Alpha Sapphire. Rayquaza does not require a Mega Stone to Mega Evolve, only that it knows Dragon Ascent.

Lopunny
Introduced in Omega Ruby and Alpha Sapphire.

Garchomp

 * Garchompite Location: Found near structure in Victory Road (post-game only).

Lucario

 * Lucarionite Location: Held by Lucario from Korrina.

Abomasnow

 * Abomasite Location: Given after defeating Team Flare at Frozen Cavern.

Gallade
Introduced in Omega Ruby and Alpha Sapphire.

Audino
Introduced in Omega Ruby and Alpha Sapphire.

Diancie
Introduced in Omega Ruby and Alpha Sapphire.