Mega Evolution

Mega Evolution is a concept introduced in Generation VI. It consists of certain Pokémon gaining an ultimate form of themselves. The Pokémon need special Mega Stones in order to evolve into their final form for one-evolution/branch evolution families and ultimate forms for two-evolution families. Also, Pokémon can only gain their ultimate form by using a Mega Stone unique to their species. For example: To evolve Blaziken into Mega Blaziken, Blaziken must be holding the Mega Stone, Blazikenite.

Mega Evolution does more than just change a Pokémon's appearance, it also changes their types, ability and stats. A good example is Ampharos, which after Mega Evolution becomes an Electric/Dragon-type Pokémon. But, some Pokémon, like Aggron lose a type after Mega Evolution, goes from a Steel/Rock-type Pokémon to a pure Steel-type Pokemon.

Mega Evolution is only possible during a battle or contest. After the battle/contest ends, the Pokémon will go back into their normal forms. You are limited to having only one Mega Evolution in a battle. Each individual Mega Stone can be used an infinite number of times, but only once per battle.

The Mega Evolutions are in fact their own species of Pokémon, as seen by the statement that accompanied Mega Mewtwo Y's original reveal which called it "a new Pokémon related to Mewtwo" and also Red stating in Pokémon Origins that Charizard has "evolved even further!"

Introduced in X and Y
28 Mega Evolutions were introduced in Pokémon X and Y.

Introduced in Omega Ruby and Alpha Sapphire
20 Mega Evolutions were introduced for Pokémon Omega Ruby and Alpha Sapphire.

Anime
In the special Mewtwo — Prologue to Awakening and the movie Genesect and the Legend Awakened, a Mewtwo appeared that has the power to mega evolve into its Mega Mewtwo Y form.

In the regular anime, in the first episode of the XY series, Kalos, Where Dreams and Adventures Begin!, a Mega Blaziken appeared, belonging to an unknown Trainer. It appeared again in Mega-Mega Meowth Madness!, and it was revealed that its Trainer is Meyer, the father of Clemont and Bonnie.

In the anime special, Pokémon Origins, in File 2 - Cubone, two strange stones are given to Red, not knowing what they were. In File 4 - Charizard, while trying to catch a Mewtwo, he discovered that the stones were a Key Stone and a Charizardite X, allowing his Charizard to Mega Evolve into Mega Charizard X, making him capable of catching Mewtwo.

The special Mega Evolution Special I is especially focused in Mega Evolution. First, we see a Mega Abomasnow battling a Mega Gyarados in the rain. After that, an introduction of all Mega Evolutions known at the time is given. The special is about Alain, who wants to discover the secret of Mega Evolution, battling each Trainer that can make their Pokémon Mega Evolve. First, he is battling Astrid with her Mega Absol. After an intense battle, he wins. Following, he goes and finds an Aerodactylite, and a character named Remo challenges him to a battle with his Mega Garchomp nicknamed "Garchoo" to reclaim the Mega Stone. Alain wins again with his Mega Charizard X, and following he challenges the Elite Four member Siebold. In the battle, Siebold uses his Mega Blastoise and wins.

Back to the regular anime, in the episode The Bonds of Evolution!, Ash and friends meet the Kalos Champion, Diantha, and they see her battle another Trainer using her Gardevoir with the capacity of Mega Evolving.

In the episode Mega Revelations!, Ash and friends meet Korrina and her Lucario, she explains that they're traveling to dominate Mega Evolution and then she joins them to travel. In the next episode, The Cave of Trials!, they meet Gurkinn, Korrina's grandfather. Both of them have a battle, using her Lucario and his Blaziken, respectively. The episode ends in Korrina's Lucario Mega Evolving. Unfortunately, in The Aura Storm!, when Lucario finally Mega Evolves, loses control of itself. Gurkinn then leaves them and tells her to train with her Mega Lucario. In the episode Calling from Beyond the Aura!, Korrina and Lucario train to dominate that Mega Lucario form, with no luck, Mega Lucario loses control once again. In the following episode, The Bonds of Mega Evolution!, Korrina meets Mabel and her Mawile. In that same episode, Korrina and Maple have a battle, Korrina Mega Evolves her Lucario, while Mabel her Mawile. During the battle, Mega Lucario loses again control, letting Mega Mawile win. In this episode, Korrina leaves the group.

The special Mega Evolution Special II continued with the story of Alain. The episode featured Mega Sceptile, Mega Swampert, Mega Blaziken, and Mega Metagross along with Steven Stone.

Trivia

 * Mew and Ditto can both take on the form of a Mega Evolution, but cannot Mega Evolve themselves.
 * Mewtwo and Charizard have two Mega Evolutions. The different evolutions are obtained with version-exclusive Mega Stones - for example Mewtwonite X can only be obtained in Pokemon X and causes Mewtwo to evolve in Mega Mewtwo X. The item can be traded between versions, however.
 * The Ralts family is currently the only one to have two Mega Evolutions that are a part of a family that branches off.
 * Each Pokémon that Mega Evolves gains exactly 100 points in total statistics, with the exception of Alakazam, who only gains 90 points.
 * Mega Evolution X brings a type change in Pokémon.
 * Mega Evolution was originally the transformation method, but keep the method as evolution method though to be transformation method.
 * Mega Evolution is a cross between Evolution and Forme Change.
 * Aggron is the only Pokémon to lose a type without any other replacing it after Mega Evolving.
 * Rayquaza is the only Pokémon that doesn't need a Mega Stone to Mega Evolve. Instead, it has to learn the move Dragon Ascent.
 * In the anime, Mewtwo and Diancie are able to Mega Evolve without Mega Stones.