Dragons
There are six Dragons in Restoration of Erathia.
Rampart's Green Dragon is upgraded to a Gold Dragon.
Dungeon's Red Dragon is upgraded to a Black Dragon.
The Necropolis also have access to two Undead dragons: the Bone Dragon that can be upgraded to the Ghost Dragon.
In addition, there are also 4 neutral dragons available in Heroes Chronicles, Armageddon's Blade, and Shadow of Death:
Dragons get bonuses from the artifact Vial of Dragon Blood, or being under the command of Mutare, Mutare Drake, or the Overlord incarnation of Tarnum.