Dämonfeuer

A Dämonfeuer ('alternate spelling Daemonfeuer; pl. Dämonfeuer; DAY-mon-foy-ər (Grimm: DAY-muhn-fyoo-ər); Germ. Dämon "demon" + Feuer "fire") is a dragon-like creature able to breathe fire and are among the rarer species of Wesen. They were seen in. They are renowned as dangerous and powerful, so much so that even Blutbaden fear them. They are also one of a very small number of Wesen who don't fear Grimms, nor do they seem to despise them.

Characteristics
Dämonfeuer are treasure-mongers and are very possessive of the large amounts of copper they're known to collect. They have no qualms with stealing copper to increase their horde.

Dämonfeuer, or at least male Dämonfeuer, seem to be a martial race, and have a value for combat, and see death from combat or in battle, as to die with honor and dignity, as such they are commonly soldiers, and sometimes war heroes. They also take pride from the memory of their relatives' services. For this is the reason they sometimes capture warrior’s lovers, so that the warrior would come to face them, and they would be able to fight to the death. Females also see this as an honorable death, although their idea of how they should die appears to differ. It is this which is the basis for the myths of dragon quests.

The children of Dämonfeuer are always fiercely loyal to their parent.

Dämonfeuer have a habit of making their lairs in caves.

According to Monroe, the best way to fight them is with a lance. Guns can't be used, as they would ignite the gas.

Biology
When they woge, Dämonfeuer undergo one of the more extreme changes of any known wesen. Their skin becomes a green and scaly hide, their nose becomes flat, their eyes take on a fire-like appearance, and most notably they grow a large crown of horns on their heads. Males possess a larger set of horns and have more angular faces than females. Despite rather extensive transformation, Dämonfeuer still retain physical traits of their humanity mainly their ears and hair which remain largely unchanged. Dämonfeuer are much stronger than humans, and are at least as strong as Grimms. However like humans, their strength seems to more-or-less differ by gender. Their strength and durability seems to be greatly diminished in human form, since Juliet was able to knock Ariel down with one hit, this only seems to apply to females, as Fred was able to fight Nick in his human form. Male Dämonfeuer are incredibly physically powerful, as the elderly and sickly Fred Eberhart was able to fight Nick on an equal level, and even knocked him down twice, despite Nick being much younger.

Fire and smoke production
The diet of Dämonfeuer consists mainly of humans. This fat-rich but low-carb diet induces higher levels of gastric acid production, crucial for digestion within the stomach.

The production of too much gastric acid results in frequent heartburn. If the gastric acids spill into the lungs, this induces coughing, which is the body's way of ridding the lungs of the acids that settle in them.

Dämonfeuer take heartburn to a whole new level with their unique ability to convert fat through ketosis into a highly combustible acetone vapor, mixed in with the gastric juices that spill into the lungs. Thus, when a productive cough is expelled, the combustible vapor is sprayed from the mouth. Following a series of vapor-releasing coughs, a single productive cough releasing gastric juices from the lungs is enough to ignite the gas, and giving the appearance of fire expelled from the mouth. This fiery breath causes anyone in its path extremely severe burning, and death to anyone who it hits. Their fire has been compared to napalm, in terms of heat and destruction, and is enough to melt metal and burn humans to a crisp in seconds. Dämonfeuer seem to have two forms of fire breath. The first and most used form is slowly releasing the vapor and after making the air thick with it, igniting the vapor. The other version is a quick flamethrower style attack, where the Dämonfeuer just opens it's mouth and breathes fire, like a typical dragon.

They also seem to have a natural resistance to heat and fire.

Grimm Diaries
In Aunt Marie's book, there is a story about a Dämonfeuer in medieval times. The author of the story stated that the Dämonfeuer was the fiercest of them all. He would steal all the village's treasures. Recounting the massive piles of bones near the entrance to his lair, which the author compared to an entrance into Hell.