The Monstrous Nightmare is one of the most aggressive, powerful, and stubborn dragons, surpassed only by the Seadragonus Giganticus Maximus. It is usually purple or scarlet and possesses a long, snakelike neck and tail. If you clamp its mouth shut, it can't open its enormous jaws, much like a crocodile. Moreover, the locomotion of the Monstrous Nightmare is similar to that of pterosaurs; it can use its claw-tipped wings to "crawl" along the floor. As it is quick to anger, the Monstrous Nightmare can be counted on to be the first to arrive and the last to leave in a fight. Its signature attack strategy, known as the "Fire Jacket," is to engulf itself in flames (from an external source or otherwise), resulting in an intimidating offensive and defensive maneuver similar in appearance to napalm or other gel-based incendiaries. By unofficial Viking law, only the chief, or the son of the chief, can own one. Despite this, Snotlout Jorgenson, the nephew of the chief, owns one named Fireworm, while Thuggery from the Meathead Tribe owns another named "Killer." One is used as the last initiation test for the young Vikings before the Vikings befriend the dragons. Despite their vicious nature and fierce combat abilities, it is shown that a Night Fury can defeat it through a brief but brutal fight.
The official Website describes the Monstrous Nightmare as follows: "No dragon is more ferocious, or feared than the MONSTROUS NIGHTMARE. This gigantic dragon has a long, snakelike neck and tail, and is covered in red scales and long sharp spines. The Nighmare's gigantic head and mouth can swallow Vikings whole. This dragon can attack any time of the day or night, from the air or on the ground. It is highly aggressive, and will never run from a fight. Its fire is thick and sticky, clinging to walls and running down hills like a flaming river. The Nightmare also has a nasty habit of setting itself on fire."
According to the Trivia Track of How to Train Your Dragon Blu-Ray they are 69 feet tall.
"This red-and-black creature with a huge wing span produces a kerosene gel fire. “He is the most dragony of the dragons,” Sanders says, “but the thing that we like is that he is the most showy. We always felt he was the Mick Jagger of dragons. He’s a natural-born performer who comes out engulfed in flames because they can set themselves on fire.”"