|“||It was difficult to tell because it was so dark, but it wasn't a species I recognized. It reminds me of some of the ones that I've heard about in the ancient Sagas. A Leviathorgan, or a Gorgenghast.||”|
Gorgenghasts are large, ancient sea-dragons mentioned in How to Ride a Dragon's Storm.
Gorgenghasts, like Leviathorgans, have been around since the age of the Dinosaurs. There is no mention of what a Gorgenghast looks like, but we can surmise that they are extremely large and intimidating
In Book 7, Hiccup considers the large sea dragon that is following the The American Dream 2 across the Atlantic Ocean.