For all 3 times I've played through X, when I battle Xerneas, I always managed to catch it on the first ball for all 3 times. I always use an Ultra Ball. So does Xerneas have a high catch rate? If so, what is it?
Actually, yes! Most legendary Pokémon have a catch rate of 3, but Xerneas and ayveltal have the catch rate of 45. With this being significantly higher than many others, this is probably why you were able to catch it so easily.
An interesting fact is that this is also Rayquaza's new catch rate in ORAS.