YEs, you can get Xerneas in Pokémon Y but trading is required.
Xerneas
Xerneas is #716 in the national pokedex, and it is a Fairy type Pokemon.
Xerneas, Yveltal, and Zygarde There is also Diancie, but it is an event pokemon. You get the choice of Articuno, Zapdos, and Moltres depending on your starter.
its legendary so i guess its rare
Xerneas does not have a Mega Evolution.
Xatu and Xerneas.
what is super effective to xerneas
Xerneas' name came from the letter X, the Latin word cervus (deer) and possibly from Cernunnos (a Celtic god based on a deer) and/or even Ceryneian Hind (Greek mythology).
Xerneas = Zer - Nee - Us Yveltal = Ye - Vell - Tull.
Xerneas as it is supereffective against Dark. (Yveltal is Dark/Flying)
Both the game and the TCG have any Pokemon that can use all 17 move types. Go Xerneas. (My opinion)