Well it's not impossible, Albino ism is possible in any vertebrate animal. All it is is a lacking of pigmentation due to a genetic mutation and/or missing a certain gene, or a recessive trait. So it is possible, its just that in the wild even if one does emerge those individuals do not last long. Because for one they are more sensitive to the environment, easily picked off by predators (or scene by potential prey), or selected against by other individuals of their species.