Morghul

"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

Morghul 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. Morghul was among the five dragons killed.

In the books
in the A Song of Ice and Fire novels, Morghul was a young dragon bound to Princess Jaehaera Targaryen, daughter of King Aegon II Targaryen, before the Dance of the Dragons. Morghul was never ridden. Morghul died in the Storming of the Dragonpit when it was invaded by a half-crazed angry mob after a one-handed prophet called the Shepherd began to rant against all dragons everywhere. There is little doubt that Morghul killed scores of people before being slain.

Shackled by heavy chains Morghul, it is written, was slain by a huge man who rushed headlong into the dragon's flame with spear in hand, thrusting its point repeatedly even as dragonflame melted the steel plate that encased him and devoured his flesh within. Since his real name was not known, the man became known as the Burning Knight when the tale spread.