If you find the egg somewhere random, it will be in a pokeball. But if you got the egg from breeding, it will be in one of the parents balls. How I know is because when I breeded my steenee, (female,) (great ball) and my bulbusaur, (male) (quick ball.) When the egg hatched I got a bounsweet in a quick ball. So it matters on the breeders pokeball that they came in. (It can be the mothers or the fathers ball.)