Nidhogg

Nidhogg is a Monstrous Nightmare that is seen in promotional material for School of Dragons. He appears in a promo advertising double energy bars for members.

Trained by Ruffnut
At some point before the events of School of Dragons, Nidhogg was trained by Ruffnut. As can be seen by Nidhogg's low level, it was still a baby when it was trained.

Physical Appearance
Nidhogg is representant of its species. Little can be seen of the dragon in the promotional material, but it is clearly dark red in color.

Trivia

 * Nidhogg's name is a reference to the legendary dragon of the same name in Norse mythology.
 * This dragon was said to live beneath Yggdrasil, better known as the world tree, where it would continually gnaw on its roots in an attempt to destroy it and bring about the downfall of the gods during Ragnarok, which he will survive. It is said that Nidhogg feeds on the blood of those who haven't entered Valhalla or have committed terrible crimes, that his wings are covered in corpses, and that he will fly to Midgard during Ragnarok after biting through the roots.