Threadtail

The Threadtail is a Strike Class dragon that made it's debut in How to Train Your Dragon 2, but however, had its name first revealed in Dragons: Rise of Berk.

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, Windstriker, Thornridge, Raincutter 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 this dragons nose 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 colourful dragons Threadtails have strong necks and tails and a broad wingspan built to take full advantage of the winds and updraughts. 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 develops 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.

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.

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

Trivia

 * The 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.
 * It is the first Long-necked dragon to be in the Strike Class.
 * It 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.