Nagga

Nagga is the legendary first sea-dragon in the culture of the Iron Islands. According to legend, blessed by the Drowned God, the Grey King slew Nagga on the shores of Old Wyk, then used its bones to build his hall.

In the books
In the A Song of Ice and Fire novels, the legendary Nagga was said to be so immense that it fed on massive Krakens and drowned islands when it was angered.

It has never been established that "sea dragons" were ever a real creature. In a world where dragons once stalked the skies, there may be some kernel of truth behind the legends, which were later exaggerated to make Nagga big enough to drown entire islands. On the other hand, "Nagga's Bones", the ruins of the Grey King's old hall, still exist and were used as the site where kingsmoots were held to elect the new King of the Iron Islands (before the title became hereditary). Descriptions of Nagga's Bones are vague (perhaps intentionally): according to legend it was made using the massive bones of Nagga as pillars, but the narration doesn't make it clear if these are just pillars of rock that resemble bones, or if "Nagga's Bones" actually are the bones of some immense pre-historic animal.