Shellfire

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

Official Description
"An enormous deep sea dweller with crustacean attributes, the Shellfire glides through the ocean at incredible speeds and tows the Dragon Hunter battleship bolted to its back. Reminiscent of a much larger - much angrier - horseshoe crab, the Shellfire shrugs off attacks with its durable carapace and delivers devastation with its protruding prong horns. In the past, unwitting sailors used to mistake its sizable shell for a landmass and scuttle their ships on it. That was the last time anyone ever heard from those sailors, but rumors of their demise likely gave Viggo Grimborn the idea to fasten a metal, dragon hunting warship to the Shellfire's back. The Shellfire is a gigantic Titan Wing deep-sea dragon with a flat hard back shell for protection. The Shellfire can launch huge flaming boulders that explode like giant cannonballs. Not much is known about it; Vikings of the distant past would mistake the massive shell for landmasses and wreck their ships on it."

- Dragonpedia

Physical Appearance
As a Titan Wing, 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 and a horseshoe crab, with seemingly segmented armor that coats over itself to create the carapace. The dragon has four legs and uniquely shaped wings used mainly for swimming.

Dragons: Race to the Edge, Season 4
Viggo Grimborn, his brother Ryker, and their fellow Dragon Hunters were able to capture one and used it for their secret project.

Twintuition
The Shellfire's existence is first implied in this episode, in which Viggo refers to a "Project Shellfire"; the silhouette of the dragon itself later appears at the end of the episode supporting a vessel made from dragon-proof metal.

Shell Shocked, Part 1
Having become aware of Project Shellfire, Hiccup leads the Dragon Riders in an attack on Dragon Hunter Island only to find it deserted except for an imprisoned Viggo, with Ryker having taken over Project Shellfire. The Shellfire is then used to attack Caldera Cay and the Defenders of the Wing before launching an assault on Berserker Island, at which point the Riders learn that the Shellfire is a dragon with a ship tied to its back.

Shell Shocked, Part 2
Ryker subsequently takes the Shellfire and the Dragon Hunter fleet to Dragon's Edge, and due to the Shellfire's attack range is able to assault the riders from a safe distance while preventing them from counterattacking. Viggo is able to counter this by disturbing a Submaripper, the Shellfire's natural enemy, which attacks the Shellfire. This distraction allows the Dragon Riders to attack and free the Shellfire from Ryker's control.

Strength and Combat
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.

Judging by their size might be really powerful opponent for most opponents but their size proven to be quite slow as they won't be use their long powerful sharp two horns in combat. Their horns could be use like mammoths for main combat as a average size before the titan must be really powerful for Submaripper as natural and hated enemy.

Firepower
The Shellfire is capable of shooting plasma-charged 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's armored carapace is so hard that it 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
As a Tidal Class dragon, the Shellfire lives in deep, open ocean. It can stay submerged underwater for extremely long periods of time.

Dragons: Race to the Edge, Season 4
Viggo’s Titan Wing Shellfire make its full appearance in this season.

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. It also greatly resembles a trilobite.
 * A Shellfire's long ranged boulder blasts can be mistaken for the wrath of the gods by Vikings, such as the Defenders of the Wing.
 * As only a Titan Wing Shellfire has been seen so far, it's currently unknown what the size and appearance of a regular Shellfire is like.
 * However, it can be assumed that a regular Shellfire is about the same size as a Submaripper due to both being natural enemies.