Ah, yes, sometimes eggs received from whatever mysterious benefactor gives them to everyone will hatch into pokemon with alternate or "shiny" coloration for their species. The same sometimes occurs with eggs from breeders where one of the parents is such, though we haven't been able to figure out if there's any way to tell what causes this, or whether it can happen otherwise.

Presumably, there may be wild ones with such alternate coloration, but since many people seem to value them for being "rare", they're naturally incredibly unlikely to come across. I don't think I've seen any in the wild in my time here.