Don't have X/Y cause i don't have a 3DS, wish i had though. But i do know the shiny encounter rate was pretty much doubled.
Other shiny OC's i have are Umbreon, Ninetales, Gengar, Glaceon, Ditto, Mew
Then there are alternate colored ones, Sandslash, Shedinja, Giratina, Deoxys, Murkrow, Mewtwo, and probably a lot more i missed hehe
It was mostly to make them stand out from other Pokemon of the same species, most will be making it to my PE game that is based off PMD. I didn't make them for no reason after all :P