According to Isidore of Seville [7th century CE] (Etymologies, Book 12, 4:4-5): "The dragon is the largest serpent, and in fact the largest animal on earth. Its name in Latin is draco, derived from the Greek name drakon. It has a crest, a small mouth, and a narrow throat. Its strength is in its tail rather than its teeth; it does harm by beating, not by biting. It has no poison and needs none to kill, because it kills by entangling. Not even the elephant is safe from the dragon; hiding where elephants travel, the dragon tangles their feet with its tail and kills the elephant by suffocating it. Dragons live in the burning heat of India and Ethiopia. (Book 16, 14:7): Dracontites is a stone that is forcibly taken from the brain of a dragon, and unless it is torn from the living creature it has not the quality of a gem; whence magi cut it out of dragons while they are sleeping. For bold men explore the cave of the dragons, and scatter there medicated grains to hasten their sleep, and thus cut off their heads while they are sunk in sleep, and take out the gems." Source

There's no question of flames coming out of his mouth and nostrils in this description. But in L'Encyclopédie du Merveilleux : Du bestiaire fantastique it is said that dragon comes from the depth of the Earth and breathe Hell's fire.

The Devil is likened to a dragon because he is the worst of all serpents. As the dragon makes the air shine, so the Devil makes himself appear as the angel of light to deceive the foolish. The crest of the dragon represents the Devil crowned with pride. As the dragon's strength is not in its teeth but in its tail, the Devil, deprived of his strength, deceives with lies. The way in which the dragon attacks elephants represents the way the Devil attacks people, lying in wait along their path to heaven, wrapping them in his coils, and suffocating them with sin. Source

In L'Encyclopédie du Merveilleux : Du bestiaire fantastique: "The European dragon is demoniac and malicious."