Common Egg can be bought for 1 Crystal from the Get Pets page. This kind of egg will hatch either C or B grade Pets, or possibly 1,000 coins if you already have at least one C or B grade pet at Level 8. You will also receive 3 gift points for each purchase. You may receive an upgrade of a Pet you already own, if it's not at level 8 yet.
List of Pets obtainable or upgradeable from Common Egg Edit