Quicksilver

Quicksilver was a dragon belonging to Prince Aegon Targaryen.

History
Prince Aegon rode Quicksilver in the battle beneath the Gods Eye, during which it was killed by Balerion the Black Dread.

In the books
In The World of Ice & Fire and Fire & Blood, Quicksilver first bonded with Aenys Targaryen. Aenys, who had been sickly from birth, quickly became healthy after bonding with the dragon, ending the rumor that the prince was a bastard fathered by someone other than King Aegon.

Aenys flew Quicksilver to attend a feast in Sunspear celebrating the tenth anniversary of the peace between Dorne and the Iron Throne. Aenys was at Highgarden with Quicksilver when he received the news of his father's death, and flew the dragon to Dragonstone to attend the late king's funeral. When Harren the Red rebelled at Harrenhal, Aenys and Quicksilver were at Riverrun. Lord Tully urged the king to descend on Harrenhal with Quicksilver as his father did with Balerion, but he refused.

After Aenys's death, his son Prince Aegon bonded with Quicksilver. Quicksilver died in the Battle Beneath the Gods Eye when Balerion ripped a wing from its body mid-air.