Fireworm
Lanthanodrax speleotraxis.1 (Also the occasionally occuring "sport" or hypotrophic subspecies, L. speleotraxis ignigastris.)
The fireworm is possibly a candidate for the single North American species which has had its evolution most seriously interfered with by wizardry. The original species -- just another small reptile, to start with -- was repeatedly mutated by the wizards caught up in the events surrounding the final fate of Afallonë. The most comprehensive change to their structure happened as a side effect of the great aphthonic intervention, the wizardry intended to save as much as possible of the unique Atlantean flora and fauna from the coming destruction of the First Continent. The "rafting" wizardry which was meant to simply transport a number of species to the newer continents went wrong during the final crisis in many important ways, and rampant wizardry leakage contaminated and disrupted the genetic structures of many of the creatures involved. The fireworm, already susceptible to such disruption because of previous genetic manipulation, suffered far more of it during the transport to the early versions of North America and Europe than any other creature involved in the ancient rafting project.
In Europe and Asia, the fireworm had fewer natural enemies, and grew and changed in ways which would otherwise have been impossible. Further mutations due to the accumulation of wizardly overlays in the more populated parts of Europe led to the rise of the offshoot species Draco, the "true dragons" which became famous in Europe's medieval period for being killed by knights. (It should be made clear that most of these casualties were members of the species Draco ectenis, the "lindworm" or wingless dragon, an omnivore by habit and cave-dweller by preference. Even the most heavily armored knights had a terrible batting average against Draco draco Europaeis, the cliff-dwelling, strictly carnivorous winged dragon.)
But in North America, where many "rafted" species with wizardly components survived for the next few millenia after the fall of Atlantis, the fireworm remained for some time as it had been in Atlantis -- a small carnivorous reptile similar to a caiman, with a tendency to burp flammable gas. (No dragon, true or false, breathes fire: but even the smallest ones can ignite it on exhalation in one of a number of ways, usually involving chemical or enzymatic reactions.)
1The taxonomy of the western European fireworm is somewhat confused. The Bestiarium Ignotum classified the fireworm as Lanthanotrops micrognotus, but this did not take into account the internal differences between the Lanthanotrops and Lanthanodrax families -- possibly understandable, since Linnaeus was unable to find a dead specimen to dissect, and trapping a live one was at that point out of the question. The Materia Magica's classification group has now placed the fireworm among the various members of family Hemerothalcus, with the "true dragons" of the Lanthanodrax and Draco species.