Fathomfin

The Fathomfin is a massive Tidal Class dragon that is first seen in The Serpent's Heir. Its name however is revealed in the upcoming The Fire Tides.

Hatchling to Adult
The Fathomfin is a massive dragon. Its body is mainly blue with green patterns and it has a massive fin decorating its back. It also has barnacles attached to its body.

The Serpent's Heir
When the Dragon Riders are about to reach Nepenthe, Hiccup uses his spyglass to spot a Fathomfin surfacing in the distance. Fishlegs got really excited about the new dragon speceis, but Hiccup tells him that studying the Fathomfin will have to wait.