As far as I'm aware, Arcanine is obtainable in both versions.
X exclusives: Starmie, Pinsir, Houndoom, Mightyena, Aggron and Sawk
Y exlusives: Cloyster, Heracross, Tyranitar, Manectric, Liepard and Throh
None of which matters anyway, because MegaGengar with Shadow Tag is going to be...