An (dragon) egg refers to an unhatched Dragon. You can receive new dragon eggs by Breeding. certain dragons together, winning them from events or buying them in the shop. Once an egg is bred or you claimed your egg from the shop or event you can move your egg to the Hatchery where you can hatch it to a little baby dragon.
The selling prices and hatch time of many dragons change frequently.
Some Dragons from the Deus Vault can be bought in store as of May 9th,2013, or bred, if your dragons are strong enough.
As of July 25, 2013 Dragon City Added the Light, War and Pure Dragons in The Store. Also with the new element system, Pure is now an elemental, some rare hybrids (Poo, Pirate, Petroleum) have lost their status and became hybrids.