Tyraxes

Rhaenyra Targaryen wanted every one of her children to have a dragon egg by the cradle from the day they were born. Plenty at court doubted the eggs would ever hatch, since they doubted just as much who the children's real father was. They hatched all the same.

Prince Joffrey's was named Tyraxes, and he grew bigger with every passing year.

When war broke out between Greens and Blacks, Joffrey and his cousin Rhaena Targaryen flew to Gulltown on Tyraxes's back. Their brother Jacaerys had sent them, with clear orders: keep any green dragon from threatening the Vale of Arryn.

The flight also made good on a promise to the Maiden of the Vale, who had asked for protection in exchange for her loyalty.

King's Landing fell to the Blacks, and a fleet sailed from Gulltown back to the capital, carrying Prince Joffrey and his dragon with it.

Once in the city, Tyraxes was kept confined in the Dragonpit with the other dragons. Joffrey asked to fly out and fight, but was refused.

Fear and a city with no one governing it lit the fuse for a riot, and the crowd surged toward the Dragonpit. Tyraxes tried to fall back into his own pit, burning everyone who chased him through the main entrance until the bodies blocked it shut.

The mob got in through a back way instead. Turning to face them, the dragon tangled himself in his own chains and was left helpless before his attackers. Once he was down, they cut his wings apart with knives to make cloaks from the hide.

Prince Joffrey died that same day, trying to reach the Dragonpit to save his dragon.