The skrill

the skrill
 * "If you get too close to a Skrill, your hair will stand on end."
 * ―Hiccup, reading from the Book of Dragons.

The Skrill is an elusive dragon appearing in the film adaption of How to Train Your Dragon. Accordng to the Book of Dragons, it is one of the most mysterious and formidable dragons. It is also one of the most powerful of dragons, rarest, and is the second most intelligent and fastest dragon. The Skrill is seen in the Dragon Manual while Hiccup looks for more information on Night Furies. A frozen specimen appeared in A View to a Skrill Part I  and II.

Dragon Appearance
This incredible creature has large wings, a spiked back and a tail. The spikes on the Skrill's back and tail are sharp enough to cut with the slightest abrasion, and it's an excellent flyer. It also appears to have shark-like gills on its neck. Its head slightly resembles a Thunderdrum, and its tail also resembles that of a Deadly Nadder. Proportion of head, trunk, and wings are quite close to that of a Flightmare. It apparently resembles a Night Fury in flight and rivals its speed, being only slightly slower. It's the size of a Nadder, and incredibly fast, comparing speed to size.

When a Skrill is riding lightning, it seems to wrap itself in its wings and dive down. Like the Monstrous Nightmare, its locomotion on land resembles that of a pterosaur, using the large claws on its wings to walk and run on land. Find more information at Dragon Anatomy.

As with most of the dragons in the manual, it's listed as being "extremely dangerous" and the manual instructs readers to "kill on sight".

Behavior
This dragon extremely aggressive. They like to eat sheep, and they often fly in thunderstorms so that they can recharge their shot limit quickly. This dragon is rarely seen due to its reclusive nature, not unlike the Night Fury. They also seem to be fairly easy to fool, though they are not stupid. The Skrill's very high aggression levels mean that it is nearly impossible to train, but is still possible when having a lot of patience.

Abilities
The Skrill electrifies itself when hunting. It also rides lightning bolts to reach supersonic speed. The Skrill is only found during lightning storms. It can breathe out concentrated beams of powerful electrical energy. Using the metal-like spines on back as conducters, Skrills are able to store lightning in their bodies and use it later. In the waters, however, they cannot shoot lightning bolts and that are the moments when this deadly species can be vulnerable. They can also survive for decades when frozen due to their inner body temperature. Their powerful electrical energy can make Vikings talk in a strange language (if survived). They have shown great stealth in the air able to be able to pass the dragon riders. The Skrill has a strong sense of vision that enable it to see in the dark and in dark clouds.

How To Train Your Dragon
The Skrill is mentioned briefly as Hiccup reads the Dragon Manual for information on Night Furies.

In the Book of Dragons
The Skrill is featured in Book of Dragons. It is said to be highly secretive and is known to ride lightning bolts like air torpedoes to reach supersonic speeds, and usually only comes out during electrical storms. It can shoot blasts of lightning as its entire body crackles with static electricity. Just being close to a Skrill is enough to make one's hair stand on end. Skrills belong to the Strike Class of dragons.

Dragons: Defenders of Berk
The Skrill appears in Defenders of Berk in episode 10 and episode 11- View to a Skrill Part 1 and 2. In the episode, the Skrill is found frozen in a solid block of ice, which eventually, the twins and Snotlout blow up, unleashing the Skrill. It is pursued by the gang for almost the whole episode and in the end is captured by Alvin. In part 2, the Skrill is locked up and put in a cage on Outcast Island. Wanting the dragon, Dagur secures a deal with Alvin. In the end, the Skrill is refrozen in an glacier once again.

Trivia

 * The Skrill is the Berserker Crest.
 * At full speed, the Skrill is the 3rd fastest in the franchise, with the first being Night Fury's, and the second one being the Speed Stinger.
 * The Skrill has an error on it's official page (in the gallery). Instead of saying it is in the Strike Class, it is labeled as being in the Sharp class.
 * According to the Cartoon Network site, Skrills are completely untrainable. Dragonpedia, however, says Skrill are nearly untrainable which is the case.
 * A Skrill's favorite food is sheep(mutton).
 * The Skrill made it's first appearance on the series in the episodes View to a Skrill Part I and View to a Skrill Part II.
 * The Berserkers can actually control a Skrill to shoot out lightning during electrical storms.
 * The Skrill is said to draw lightning from the sky and strike it in several directions at once.
 * Due to the Skrill's internal body temperature, the dragon can remain safely frozen for decades.
 * In A View to a Skrill Part I, Hiccup seems to have great interest on the Skrill along with Fishlegs.
 * It is possibly the first time that Hiccup gets so excited on seeing a dragon.
 * The Skrill in the Book of Dragons and Riders of Berk's trailer seem to shoot different type of lightning to the one on Defenders of Berk. This may be caused that the Skrill on the Book of Dragons and RoB trailer is the original Skrill, which shoots white fire while the DoB one is a developed Skrill that shoots true, long distanced lightning bolts.