The Flightmare is a Mystery Class (originally and formerly from the Strike Class) dragon that appears in Dragons: Defenders of Berk. It is among the most terrifying and frightening dragons. It appears in the episode Fright of Passage.
The Flightmare is a medium-sized dragon that has a very ghost-like appearance. Its body is covered in small spots that almost resembles the night sky. It slightly resembles a mix between a Skrill, a Night Fury, and a Fireworm, with glowing scales. It appears to get a brighter glowing, ghost-like appearance from the glowing blue algae that is the staple part of its diet.
From what has been seen, the Flightmare is a very aggressive and territorial dragon, being very protective of it's territory and food sources. Seeing how Flightmares are build for speed and defense, they usually avoid fights with bigger and stronger dragons. Flightmares have been shown to be quite determined, as the Flightmare in Fright of Passage kept chasing his opponents, and was willing to search food over long distances. They seem to be quite reclusive and avoid contact with humans, unless, they get in the way of their algae, or if they think that they do.
The Flightmare's body glows by eating algae that shines along with Arvindal's Fire, a type of the Northern Lights that can only be seen every ten years. Arvindal's Fire is a greenish blue, teal color. They can use this ability to make themselves brighter against their prey or enemies.
The Flightmare can strike with a mist that paralyzes its prey to give itself enough time to return and finish it. In order for the Flightmare to kill its prey with its mist, it needs to spray its prey for a few seconds in order for it to freeze to death, while the temperatures reach below freezing.
Flightmares have shown to be fast and agile, as the specimen in Fright of Passage was able to catch up with Toothless with ease. Even so, when Toothless was trying to get away from the Flightmare, it quickly boosted in front of him.
They have great stamina as shown by Hofferson's Bane, as it was able to fly long distances as it flew across the ocean to follow the glowing algae.
They can produce a loud banshee-like scream that can slightly affect other dragons.
In Dragons: Defenders of Berk
The Flightmare first appeared in Fright of Passage, terrorizing Berk every ten years. Last time, Astrid's uncle, Fearless Finn Hofferson, faced the dragon and seemed to be frozen by fear, shaming her family's name. Astrid and Hiccup set out to find the Flightmare, where Astrid tried to fight it, but was frozen by the dragon's mist. They learned that victims of the dragon were not frozen by fear, but paralyzed. They soon discovered a stream of glowing algae that gives the Flightmare its glow, as well as other dragons (as discovered by Toothless). Realizing that the Flightmare only attacked Berk because it thought the Vikings were threatening its food supply, Hiccup, Astrid, and Fishlegs created a new channel for the algae to flow into the sea, and the Flightmare left Berk. Hiccup explained to the village what they learned about the dragon and that Finn Hofferson truly was fearless, restoring honor to Astrid's family.
- In order for the Flightmare to kill its prey with its mist, it needs to spray it's prey for a few seconds longer, such as it did with Fearless Fin Hofferson. Whereas people who are sprayed with mist for only a short period of time can still survive, like Astrid Hofferson.
- The Flightmare's wings are translucent.
- The Flightmare is the only known dragon species that appears once every 10 years.
- The Dragon Tracker Part 3 special feature on the DreamWorks Dragons: Defenders of Berk Part 1 DVD (released 3/25/2014) reveals that the Flightmare is a Strike Class dragon.  Oddly enough, the new book Guide to the Dragons and the Dragonpedia reveal that the Flightmare is a Mystery Class dragon.
- The Flightmare's design is similar to that of the Skrill, resembling that of Pelican Eels.
- It is the only dragon that glows other than the Fireworm, and both glow via eating something.
- The Flightmare is an herbivore, because it eats glowing algae.
- When the Flightmares was first seen in the Defenders of Berk trailer, many fans thought it was a Skrill.
- The Flightmare is one of the three dragons that doesn't have a forked tongue, along with the Woolly Howl and the Snaptrapper.
- The Flightmare is now in School of Dragons as a dragon that can be obtained from the hatchery.
- The Flightmare that attacked Finn Hofferson made an appearance in Dragons: Rise of Berk and is now known as the Hofferson's Bane.
- The Flightmare's mist design is very similar to the original Skrill's lightning.
- It's glowing algae only glows every ten years around Berk, when Arvindal's Fire appears above the island.
- The Flightmare has the anatomy and look of a Strike class dragon, but most of it's abilities are those of a Mystery Class dragon.
- Almost all bioluminescent algae is marine based. So, the Flightmare should have been living by the sea.