Sea dragons

"Blessed by the Drowned God, the Grey King fought and slew Nagga the great Sea Dragon and took her fire for his own."

- Theon Greyjoy Sea dragons are massive reptiles which were once native to the seas of The Known World. They are similar to their mainland counterpart, dragons, excluding the fact that they are wingless.

History
During the Age of Heroes, the sea dragon Nagga was killed by the first King of the Iron Islands, the Grey King.

In the books
Sea dragons are possibly mythical creatures of the sea. Some maesters question their existence, as they have not been seen in millennia. Believers think they still exist deep in the Sunset Sea.

It is unknown if sea dragons are related to dragons, although Sea Dragon Tower is a dragon-shaped tower at Dragonstone in the narrow sea. Sea Dragon Point is a peninsula of the north in northwestern Westeros.

According to ironborn legend, Nagga was the first and mightiest sea dragon, a creature which fed on krakens and leviathans. She was slain by the Grey King, however, and her bones formed part of the Grey King's Hall.