Cofagrigus and Yamask are not obtainable in Pokémon X and Y, and thus you either have to transfer them from any Generation V game (Pokémon Black, White, Black 2 and White 2) through the Poké Transporter and Pokémon Bank 3DS apps, ask someone to trade them with you or look for them at the GTS.
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.
You can catch Mewtwo at the Unknown Dungeon after beating the Elite Four and entering the Hall of Fame.
Yes You Can Get A Pikachu In Pokemon X and Y.
In terms of Legendaries, most of the ends of the caves in Pokemon X and Y (Like the Sea Spirit's Den in the Azure Bay near Korrina's town
Mew is not catchable in Pokémon X or Y. You must either trade for it or transfer via Poké Transporter and Pokémon Bank from an older game.
You can catch Ditto in the Pokemon Village.
Someone hacked the game and found Mew in the code, but it is currently not possible to get it (without hacking).
The Legendary Pokemon available in Pokemon X/Y are:Xerneas (X only)Yveltal (Y only)ArticunoZapdosMoltresMewtwoZygarde
Pokemon X and Y are sold at MSRP of $39.99 (CAD)
In Pokémon X and Y, you can catch Skiddo at the purple flowers around Route 5.
There are some Kalos region pokemon available in Omega Ruby and Alpha Sapphire such as Skrelp and Clauncher. More are available in Pokemon X and Y.
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