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 dark green patterns and it has a massive fin decorating its back. It also has barnacles attached to its body, similar to real-life baleen whales.

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 species, but Hiccup tells him that studying the Fathomfin will have to wait.