The Draccus is what most people commonly mistake for a dragon. Large, heavy, and for the most part reptilian, they are the center of most dragon mythology.
While they can breathe fire, they look almost nothing like dragons of traditional lore, instead resembling a large, scaly hippopotamus. Their diet is strictly herbivorous, consisting mainly of trees, and as such they live in densely forested areas. They are by no means confined to one area, instead roaming around the general vicinity they live in. Their ability to breathe fire, combined with their choice of habitat, would, as one may imagine, result in some difficulties, had the Common Draccus not learned to put out fires by dousing them with its enormous body.
The Draccus has scales with a vast amount of iron, this is believed to be due to the rocks the Draccus consumes to help digest the trees it feeds on. The iron from the rocks is absorbed into the scales, and when shed is eaten by the Draccus to keep the iron within it. The Common Draccus is best detailed in The Mating Habits of the Common Draccus, by Devan Lochees.
The draccus that Kvothe and Denna met while hunting for Master Ash in the forest surrounding Trebon was addicted to denner resin. While hiding from the massive, fire-breathing, cow-lizard, Denna mistook raw denner resin for syrup. While she was saved from an overdose, she had absorbed enough that she was rendered insensate with euphoria, delirium and eventually, exhaustion and unconsciousness. This forced Kvothe to leave her behind when the Draccus began to wander towards Trebon, after being attracted by the lights and bonfires of the harvest festival.
Kvothe's first attempt at stopping the beast was to tempt the creature with large amounts of raw - but highly valuable - denner resin, in the hopes that it would die from an overdose. Unfortunately, as the characters surmise, the draccus's habit of eating of burnt wood (i. e. charcoal) neutralized the drug sufficiently to prevent a fatal reaction.
With the draccus still alive and rampaging through the town, Kvothe made another attempt, using sympathy to dislodge and direct the massive iron wheel hanging from the front of Tehlu's church in Trebon. For the draccus, it proved terminally effective. For Kvothe, it brought a short coma and the loss of a valuble carcass, as the townspeople destroyed the corpse, believing it to be a demon.