Shellfire

"Tidal Class for sure. If I had to guess, Titan Wing. Bigger than a Submaripper. Long range firing capabilities."

- Hiccup

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 one of the largest dragons to appear in the series. It has two long and slightly curved horns on the sides of its head, relatively long wings, and a rough dorsal protective shell. Overall The Shellfire dragon roughly resembles a trilobite and a horseshoe crab combined, with overlapping segmented scales covering it's body forming a carapace-like armor. The dragon has four stubby legs and a pair of unique fan-shaped wings used mainly for swimming.

Firepower
The Shellfire is capable of shooting series of large plasma-charged boulders in high trajectory that have a range of over a mile. These projectiles can be fired in rapid succession like a mortar and hit like bombshells. The projectiles whistle as they fly by before exploding once they hit their target. Their firepower most likely functions under water as well, seeing as they are plasma charges, similar to that of the Night Fury, whose plasma blasts also function under water.

Strength & Combat
The Shellfire has proven to be strong enough to carry the weight of an entire ship made entire dragon-proof metal tightly strapped on it's back, as well as a large crew of Dragon Hunters with little to no effort.

Whilst in combat Shellfires use their massive body, combined with their weight and raw strength to slam against opponents, utilizing their carapace like armor and ridged spines as a defense. Although not shown on screen, Shellfires most likely use their nasal and cranial horns to ram and spar against opponents.

Shellfires have shown proficiency in combat against both Viking and dragon alike, as shown when it repeatedly used hit-and-run tactics against the Dragon Riders to avoid being seen, as instructed by the Dragon Hunters on it's back to do so. It also effortlessly defended itself against the Submaripper's blows, mostly utilizing it's upper hand in size and armor to prevent any damage, and consequently speed and deep-diving abilities to retreat once irritated.

Endurance & Stamina
The Shellfire's armored carapace is so durable and resistant that it enables it to endure most anything, even numerous blows from it's natural enemy the Submaripper, showing no signs of injury, only slight irritation.

Shellfires are also able to effortlessly travel long distances, as seen when Viggo's Shellfire traveled from island shore to island shore showing no sign of fatigue when it was under the control of Ryker Grimborn and the Dragon Hunters.

Speed & Agility
Being a Tidal Class dragon the Shellfire is a very fast swimmer, as seen in the episode, "Shellshocked", when Viggo's Shellfire was able to dive and accelerate so quickly that it repeatedly avoided being seen by pursuing Dragon Riders, and, on one occasion, disappeared into the depths of the ocean so quickly a diving Hiccup Haddock & Toothless barely managed to catch a glimpse of the dragon before it vanished. Although fast, the Shellfire was shown struggling against the smaller but swifter Submaripper, mainly due to the Shellfire's size and overlapping carapace scales leaving little room for sudden movements.

Aquatic
As a Tidal Class dragon, the Shellfire lives in deep, open ocean. It can stay submerged underwater for extremely long periods of time and is perfectly capable of hunting just about anything at extreme depths.

Behavior & Personality
As massive Tidal Class dragons, Shellfires prefer the depths of the oceans to abide and hunt, and like Seashockers and Scauldrons, they rarely rise to the surface. When they do surface however, their massive armored backs are often mistaken for landmasses and used by seafaring Vikings to dock their ships on their travels, only for the disturbed Shellfires to dive beneath the waves sinking what's on their backs with them, never to be seen again.

Shellfires are quite shy and timid by nature, preferring to avoid most battles thus relying mostly on their massive sizes, armored carapaces and the unheard of depths they call home to fend of most attacks, even from Humans and their natural enemies, the Submarippers. However, if moved to battle, they have massive cranial and nasal horn-like projections combined with their massive raw strength, powerful limbs and plasma-imbued explosive rocks they can fire like mortars to put up quite the fight.

It is unknown if Shellfires can fly, as so far no individuals have been observed airborne, though judging by their sheer size and weight, comparatively small wings and poor aerodynamic build, it is highly unlikely. Also, although undetermined, Shellfires most likely can stray on dry land, as their large and stumpy legs appear to be strong enough to support their massive weight.

Weaknesses
The Shellfire's massive size, durable carapace-like armor, and deep dwelling capabilities enable it to endure mostly anything, though due to their timid whale-like nature they tend to escape fights and avoid struggles, rendering them relatively easy to capture and exploit, such as when one individual was captured and exploited by Viggo Grimborn and Ryker Grimborn in their conflict with Hiccup and the Dragon Riders.

Shellfire's tendency to escape confrontations also enables Submarippers, their natural enemies, to often win skirmishes between the two species.

Training
It is currently unknown whether or not Shellfires can be trained, as Viggo's Shellfire, the only known individual, was so agitated by the time it got released, after being captured and forced to battle by the Dragon Hunters and the dragon's confrontation with the Submaripper that it immediately retreated to the ocean's depths, and has not been spotted since.

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.

The Shellfire's existence is first implied in the episode "Twintuition", 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.

Having become aware of Project Shellfire, Hiccup leads the Dragon Riders in an attack on Dragon Hunter Island in "Shell Shocked, Part 1" 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.

Ryker subsequently takes the Shellfire and the Dragon Hunter fleet to Dragon's Edge in "Shell Shocked, Part 2", and due to the Shellfire's attack range, he 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.

Dragons: Rise of Berk
The Shellfire reappears as a Legendary Dragon in the game, it unlocks Grim dragons.

Trivia

 * Viggo's Titan Wing Shellfire is the only known member of it's species.
 * It is also the only known Titan Wing member of it's species.
 * It remains unknown as to what is meant with the Titan Wing title in this dragon, since it is used as both the final stage in any dragon's life and to describe large species or subspecies of dragons in the series, a notable example being the Screaming Death.
 * 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, following all other dragon species' Titan Wing differences, it is safe to assume that Broad Wing Shellfires are smaller than their Titan counterparts, are swifter and more agile, sport lighter colors, and possibly posses the ability to fly.
 * The Shellfire's horns resemble a Bewilderbeast's tusks.
 * Besides the obvious color difference, the Shellfire's plasma charges bear a striking resemblance with Lord Shen's Cannon ammunition seen in Kung Fu Panda 2.
 * Considering that both the DreamWorks Dragons and Kung Fu Panda franchises were developed by the same company, DreamWorks Animation, it is very possible that the cannonball's likeness was recycled and reused to serve as the Shellfire's fire type.