The Skull of Balerion
The greatest dragon of the Targaryens, and the one King Aegon the Conqueror flew when he defeated and united the seven kingdoms. He lived long enough to be ridden by Viserys I, but was old enough to die well before Viserys ascended to the Iron Throne. (This skull was still in the catacombs of the Red Keep during the events of Game of Thrones.)
