Threadtail

The Threadtail is a large Stoker Class dragon that first appeared in How to Train Your Dragon 2.

Official Description
"Don't touch! This dragon can secrete a poison through its skin when it senses danger. The only problem is the Threadtail is allergic to it too! It spends most of the day rolling around in Dragonberry bushes to relieve the itching. The Threadtail's unrelenting flames will destroy anything in its way."

- Dragons: Rise of Berk

Development
The Threadtail was created using a modular system to mix and match preset body parts onto different dragons. It was created from the same base that was used for the Hobblegrunt, Raincutter, Thornridge, Windstriker, and two other unknown dragons.

Egg
Similar to a Snifflehunch's egg, a Threadtail's egg has several lobes at its side. The eggs are usually cyan, speckled with brown spots. The color of the egg also defines the color of the dragon.

Hatchling to Adult
This dragon seems to always have bright colors such as bright pink and purple, deep blue with bright green or pale blue with bright yellow. The reason for this colorful appearance is most likely the same as the reason behind the poison dart frogs' bright colors: to warn predators of its poisonous skin. The Threadtail also has a large round frill at the back of its head, and this frill is outlined with round spines which gives it a very crown-like appearance, and it is the same kind of spines can be seen on the nose of this dragon and running down its neck and back. In the movie the Threadtail is seen to have skin covered in many round bumps and lumps that look very similar to something like frog warts, this is most likely a reference to its venomous skin and its allergy to it, meaning those might be caused by scratching.

This dragon's overall body shape seems to be most similar to that of the Thornridge. Long and often colorful dragons, Threadtails have strong necks and tails, as well as a broad wingspan built to take full advantage of the winds and updrafts. They have a prominent frill on the back of their long-jawed head and a ridge of spines along their backs and tails. Their legs are slender and built for running with shorter and denser claws than many dragons of similar size.

Titan Wing
Titan Threadtails turn into a bright shade of orange and tan. Their crowns develop sharper lobes, and more spines line down their back and snout. The spots on their wings also fade, giving way to less distinct stripes radiating from its wing bone.

Poison
The Threadtail is the only known dragons that can produce toxins through its skin to ward off predators in the franchise. This defense mechanism comes with a price, for the poisons trigger an allergy and cause the dragons' skin to itch and they can have roll around in Dragonberry bushes to relieve the itch.

Firepower
The Threadtail can breath normal, yellow fire, which can be held in its mouth to illuminate dark places. It can also produce green fire which does not come out in powerful flames, but instead spreads out slowly, like a mist or a fog.

Purple Mist
In addition to its flames, the Threadtail is known to emit a purple mist, A particular individual, Mildew's Strain, breaths purple mist and leaves puddles of shimmery purple liquid on its stand in Rise of Berk. The color of the liquid appears to match that of the Dragonberry bushes surrounding it.

How to Train Your Dragon 2
While numerous Threadtails have been seen lighting up caverns and soaring about Valka's Mountain, no individual Threadtails, nor the species, appears to have much significance in the plot.

All the Threadtails from Valka's Mountain fought viciously in the battle against Drago's Army. After Drago's Bewilderbeast defeated Valka's, all the Threadtails bowed to their new Alpha. After they were freed by Toothless from the Bewilderbeast's control, they fought against the giant dragon. In the aftermath, they bowed to Toothless, acknowledging him as their new Alpha.

Dragons: Rise of Berk
The Threadtail's name was first revealed in this game, along with its class and other background information. The individuals, Mildew's Strain and Foliander, also became available in the game.

Trivia

 * Though first seen in How to Train Your Dragon 2, the Threadtail's name was not revealed until 2016 in Dragons: Rise of Berk.
 * The Threadtail's Class was erroneously stated to be "Striker" on Facebook in 2016 at its release in Dragons: Rise of Berk.
 * The Hobblegrunt model appearing in the game School of Dragons is extremely similar to the Threadtail, rather than the Hobblegrunt model appearing in How to Train Your Dragon 2. This is a huge source of fan discontent, which is seen as an error on the game's part. However, a School of Dragons Narrative Designer, Brian Yoon, clarified that the game submitted two models to Dreamworks for approval and the Threadtail-like model was selected and named. There is no plans currently to alter the name or dragon model in the game.
 * The Threadtail's ability to secrete poison through its skin is similar to the poison dart frog.
 * The Threadtail is the fourth dragon to have a head frill, the first being the Red Death, the second being the Hobblegrunt and the third being the Thornridge.
 * The Threadtail is the second dragon after the Fireworm that is allergic or scared from something secreted from it's body. The Fireworm can light itself on fire and is also scared of it too.