You can catch Mewtwo at the Unknown Dungeon after beating the Elite Four and entering the Hall of Fame.
Mewtwo can Mega Evolve into either Mega Mewtwo Y or Mega Mewtwo X.
*Mew is not catchable in Pokemon x and y, but you can get him online from wondertrades and such. Mewtwo however, is catchable and you can find him in the cave in pokemon village.
Mewtwo X is better at Attack and Defense than Mewtwo Y.
No, Mewtwo cannot evolve into Mewthree. Mewthree isn't an existing Pokémon however Mewtwo can use a Mega Stone in Pokémon X and Pokémon Y in order to mega evolve during battle into MegaMewtwo.
Mewtwo is the ultimate Psychic type legendary Pokemon. It was introduced in the first generation and does not evolve into any other Pokemon, although as of the sixth generation it can mega evolve into Mega Mewtwo X or Mega Mewtwo Y.
MEWTWO is a very strong Pokemon with the 2nd highest sp atk in the game!!! Its speed is good aswell!! In the Pokemon first movie mewtwo is enemy and against mew but mew beat him so they are both good.
Mewtwo cannot evolve, but it can mega evolve temporarily in Pokemon X, Y, Alpha Sapphire, and Omega Ruby by letting it hold a Mewtwonite X or Mewtwonite Y.
It reappears after defeating the Elite four again.
Xerneas (Exclusive to X) Yveltal (Exclusive to Y) Mewtwo Zygrade Zapdos (Only if you picked Fenniken) Articuno (Oly if you picked Chespin) Moltres (Only if you picked Froakie)
The Mewtwonite X is exclusively for Pokémon X while the Mewtwonite Y is exclusively for Pokémon Y, so in order to get both Mega Stones (or more of the same one) and another Mewtwo in a single game, trading is required.
You can't unlock Mewtwo in Diamond, because Mewtow can't be found in Diamond. The only way to get it in Diamond is to trade or migrate it. You can migrate it from FireRed/LeafGreen and then catch it in the Field area in Pal Park. You can also trade it from someone else or from HeartGold/SoulSilver and X/Y.
Aron is unobtainable in Y, but you can use the GTS to trade one for a larvitar, the Y counterpart to a user of X.