That's the question posed by Bjorn Carey of Popular Science. He consulted Jack Conrad, a paleontologist, who says that the basic components of a dragon are available in the animal world:
In Conrad’s opinion, the leathery wings of a pterosaur are the best possible flight mechanism for a giant lizard. “Quetzalcoatlus had a 30-foot wingspan,” he says. “That would do the trick.” Big, strong wings are necessary to compensate for the weight of a dragon’s skin, which, of course, would need to withstand bow-and-arrow attacks. “Let’s throw a little alligator in there for armor,” Conrad says. An alligator’s skin, he explains, is made partly of bony plates. When European settlers first encountered the reptiles, the skin proved to be tough enough to turn away a musket ball, plenty strong for a dragon.
OK, so we’ve got a very large alligator with the wings of a pterosaur that can repel musket fire. Now it just needs to breathe flames. This is where no parallel exists—there are no known animals that can spit fire or even a flammable liquid. But there are some beetles that can shoot caustic chemicals from their abdomen that can burn people’s skin, so it’s not totally out of the question that some animal at some point in time could make a flammable liquid. Cobras can spit venom with great accuracy at objects six feet away; the dragon could borrow that ability to propel the flammable liquid. As for lighting it? “Well, maybe, if you had some specialized organ like an electric eel’s tail dangling in the mouth, that could spark that liquid and allow the creature to breathe fire,” Conrad says. “Of course, this is all very theoretical.”