Lycanwings resemble a combination of a werewolf and a dragon. Only minimal information on them has been recorded; these dragons are extremely rare, and most believe they are a work of fiction. According to the small amount of knowledge we have on these creatures, anyone bitten by these dragons eventually turn into one.
Because few sightings of these beasts have ever occurred, they have been categorised into the Mystery Class.
In the HTTYD:RTTE (How to Train Your Dragon: Race to the Edge) series you can see the Lycanwing pop up in many cases e.g. from the Dragon Eye and even when it was suspected Tuff Nut had been bitten by one.
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Lycanwings are roughly 3 times the size of a titan wing monstrous nightmare. This dragon is strong enough to bite through grockle iron wings larger than the average vikings house. According to the dragon eye, the victims of the Lycanwing's bite turn into a lycanwing. However, it was later discovered that this was made up by dragon hunters to scare people away from the dragon eye lenses hidden on Lycanwing Island.