Shellfire

The Shellfire is a gigantic Tidal Class dragon that was introduced in "Shell Shocked, Part 1".

Description
"The Shellfire is a gigantic Titan Wing deep-sea dragon with a flat hard back shell for protection. Not much is known about the Shellfire; Vikings of the distant past would mistake the massive shell for landmasses and wreck their ships on it. It can launch huge, flaming boulders that explode like giant cannonballs."

- Official Site

Physical Appearance
The Shellfire is very large. It has two long horns on its head, relatively long wings, and a hard shell on its back for protection. Overall the dragon resembles a trilobite, with seemingly segmented armor that coats over itself to create the shell. The dragon has four legs and uniquely shaped wings used mainly for swimming.

Strength
The Shellfire has proven to be strong enough to carry the weight of an entire iron ship that is tied up with large wood under the ship, as well as many Dragon Hunters, on its back with little to no effort.

Firepower
The Shellfire is capable of shooting energized boulders that have a range of over a mile. These projectiles can be fired in rapid succession and hit like bombshells. The shots whistle as they pass by before exploding like mortars once they hit the ground.

Endurance and Stamina
The Shellfire can take a series of attacks from it's natural enemy, the Submaripper, and is still able to continue to fight against it. It is also able to travel long distances, as seen when one was under the control of the Dragon Hunters.

Speed and Agility
The Shellfire is most likely a very fast swimmer. This is because in the episode, "Shellshocked", the Shellfire dragon was able to swim deep and fast enough, in just a matter of seconds, that Toothless couldn't swim deep enough to reach it. Though the speed of this dragon might not be as fast as the Submaripper as it is smaller and slightly more agile then Shellfire.

Aquatic
It seems this dragon lives in the deep ocean. As a Tidal class dragon, it can submerge underwater for extremely long periods of time.

Dragons: Race to the Edge, Season 4
A Titan Wing Shellfire made an appearance in this season's finale.

Trivia

 * The Shellfire is the natural enemy of the Submaripper.
 * The Shellfire appears to be based off of a species of Eurypterid or sea scorpion, a prehistoric sea creature with a segmented shell.
 * A Shellfire's long ranged boulder blasts can be mistaken for Ragnarok,  the wrath of the gods, by Vikings.