Flying dragons are creatures of fantasy, or at least that is what most people today think, but one must ask, could they have possibly existed? It is easy to say “NO” without backing the answer up to any degree. Let us talk about facts.
When we look at the old depictions of dragons we see they take different forms but were common in legends across all of Europe and Asia. Most depictions show a four-legged beast, some with wings, some without, we are looking at the winged animals. Their bodies usually are either long and serpent like or more robust. The word dragon itself traces to the ancient word Draco which in Roman times referred to any massive snake such as the python.
When we ask ourselves if these animals actually existed we must look at physical shape. We today know of no mammals, birds, reptiles, or amphibians, with more than four limbs. As such it is highly unlikely that any dragon would have existed having six limbs (four legs and a pair of wings). This assembly of limbs would require a very different skeletal structure than the one most animals operate on. Of course with no skeletal evidence it is impossible to say without certainty that they didn’t not exist in this form, but it is very likely this shape is a total fabrication.
The more likely form flying dragons took, if indeed they did exist, would have been two hind limbs and wings for the front. This is the same build that modern day birds have and ancient Pterosaurs (ancient flying beasts that lived from the late Triassic to the late Cretaceous era) took. The name given to this group of flying dragons is Wyvern.
To say without certainty that Wyvern never really existed is a bit more difficult than saying the other from of dragon did not exist. After all, nobody had a camera or YouTube back in those days. Science has found no evidence that these animals did exist. There are no bones, no teeth, no scales… nothing. Not even teeth or claw marks on castle walls. You would think if such a creature did exist and played such an important role in legend a skeleton would be kept as a prize of a kill.
A flying dragon would need a place to land, to take off, and to nest. The mountains offered all of these, but food would be tougher to find, assuming they were carnivores. Prey animals, such as deer, could easily hide in the forest to avoid getting snapped up. An animal with a large wing span, as surly a flying dragon would have, could not pursue prey into the woods. This left only two main possibilities. Either flying dragons were fish eaters, scooping fish out of the seas and lakes, or they ate prey that lived out in the open, such as domesticated livestock. Either scenario would put them in positions to be witnessed by people, either fishermen, or farmers. They would have very likely been recognized as dangerous predators and would have been hunted extensively as such, not just by a few brave knights.