Dragons are "a fabulous monster variously represented, generally as a huge, winged reptile with crested head and terrible claws and teeth, and often spouting fire." as defined by Webster's Ecyclopedic Unabridged Dictionary of the English Language. This being true, as is usually the case with Webster's Dictionary, it only begins to scratch the surface of what a Dragon is.
Dragons are creatures which almost any person in the world knows something about. Whether they believe it to be or have been a real living creature, a spiritual creature or simply a myth and nothing more. Every major culture has some mythology with a form of Dragon in it and the simple truth is that Dragon's are more widely known than many other currently existing species of animal.
Dragons can be dated back far beyond even recorded history. There are cave drawings of giant lizards spouting, what seems to be, fire. Many scientists have written this off as early man's interpretation of the flicking of a lizard's tongue, yet the recordings of Dragons do not end there.
Later, in Ancient Egypt there were hyroglyphics showing a 'dog-like' lizard which stood waist high which had completely straight vertical legs instead of legs which are bent to a right angle at the knee like conventional lizards. This as well could be passed off as missinterpretation.
Still later on, and up until just recently by historical standards, most people were only familiar with their surroundings and knew little of the outside world. Sailors would return from voyages and share strange stories of the creatures from far off lands. Scholars would gather this information and form huge, illustrated tomes of these creatures, such as Pliny's Natural History. The Ancient Greeks also practiced this, including the species we've heard about in Greek mythology like the Cyclops and the Rocs. Greek writers such as Ovid and Demosthenes claimed the seas were full of Sirens. Plutarch of the Romans claimed a Faun was captured and brought to Rome. Even St. Augustine claimed to have seen Fauns. One thing that most of these books had in common was that they never left out the species known as Dragon.