Syrax

"When Targaryen fought Targaryen in the civil war called the Dance of the Dragons, an angry mob stormed the Dragonpit, that huge now-ruined vault where Targaryens stabled their beasts. Thousands died, but through sheer numbers and madness, five of the Targaryen dragons lay dead by the morning."

- Pycelle

Syrax was a dragon bred by House Targaryen.

Biography
During the Dance of the Dragons, an angry mob of smallfolk stormed the Dragonpit where House Targaryen stabled their dragons. Shrykos was among the five dragons killed, being slain by a mob of attackers.

In the books
In the A Song of Ice and Fire, Syrax was a she-dragon ridden solely by Rhaenyra Targaryen.

Syrax had yellow scales. She was huge and formidable, but not as fearsome or experienced in battle as Caraxes. She was kept in chains and exceedingly well fed. She had not hunted for years at the time of the civil war.

Rhaenyra first took Syrax as a mount in 104 AC, when the princess was seven years old. Syrax was described as a "young" dragon at the time, and Rhaenyra herself named her "Syrax" (after a Valyrian goddess) - strongly implying that Syrax had no previous rider.

Syrax laid "several" clutches of eggs during the reign of Viserys I. Her latest clutch was produced just prior to the civil war. One of the eggs from that latest clutch was given to Rhaena, Rhaenyra's step-daughter.

Following her difficult stillbirth at the beginning of the war, Rhaenyra recovered enough strength to fly on Syrax, along with her other dragonriders, when they assaulted and captured King's Landing, with little opposition.

In King's Landing Syrax was kept within the walls of the Red Keep in a stable in the outer ward that had been emptied of horses and given over for her use. Heavy chains bound her to the ground. The chains were long enough to allow her to move from the stable to yard but kept her from flying off riderless. The three other dragons belonging to the blacks were kept in the Dragonpit along with the greens's Dreamfyre.

Syrax was slain in the aftermath of the Storming of the Dragonpit. Prince Joffrey Velaryon had attempted to mount her to ride to the Dragonpit, save the other dragons, and perhaps mount his own dragon Tyraxes there. Dragons are not horses, however, and they will only accept one mount at a time. In mid-flight, Syrax shrugged off Joffrey, who fell to his death. Attracted by the bloodshed, Syrax continued on to the Dragonpit. Archmaester Gyldayn noted that Syrax could have simply stayed high in the air and made runs at the mob with her flames, but instead she descended upon the ground, killing dozens with tooth and claw. There are numerous and contradicting accounts about specifically how Syrax died, and Gyldayn does not give them in detail. What is certain is that Syrax eventually died fighting the rioters.

In the The Dance of the Dragons, A True Telling by Maester Munkun writes that Vhagar roasted Ser Byron Swann when he tried to kill her by sneaking up behind his shield so the dragon would see only her reflection - as Serwyn of the Mirror Shield did to Urrax. However, according to Tyrion Lannister, Grand Maester Munkun errs, the dragon was in fact Syrax.

That it was Syrax makes much more sense than Munkun’s version as Ser Byron Swann was the son of a marcher lord, and Storm's End was for King Aegon II Targaryen. Vhagar was ridden by Prince Aemond Targaryen, Aegon’s brother and fellow green.

Archmaester Gyldayn's history of the civil war sheds no further light on the matter as it gives no account of Ser Byron Swann's attempt to slay a dragon.