Dragons are often held to have major spiritual significance. One example of them in religion would be some biblical references that seem to hint that the dragon is a type of archangel, one of which by the name of Lucifer, rebelled against God to become known as the seven-headed red dragon called Satan. Some take this as a sign that dragons are an evil force, however other parts of the Christian Scriptures (depending on the version) seem to indicate that God created multiple dragons, not all of whom rebelled. As a note, not all religions have dragons so morally in question. In many oriental cultures, they were a being to be revered, and were representative of the primal forces of nature and the universe.
Most species of dragon possessed wings. The dragons' chest was very powerful and heavily muscled. They also had a massive heart because the chest muscles needed large quantities of oxygen-rich blood to maintain a steady wing beat. The bones were perfectly designed for flight too, with an internal structure very specialized, being honey-combed, which is strong but very light weight. All of these adaptations are general characteristic of powered flight, but their wings still couldn't generate enough lift to get an animal of a dragon's size off the ground.
A dragon's wing span measured under 60 feet with an initial weight of 900 pounds. Dragons possessed a pair of leathery organs, resembling a second pair of lungs within their chest cavity. The tissue walls of these organs were thin and elastic, designed to expand. They were storage sacs. They held gas composed predominantly of hydrogen, which is 14 times less dense than air. Fill a balloon with hydrogen, it rises and when the organs were filled with hydrogen the principle was the same. The organs were the dragon's "flight bladders". All creatures contain bacteria in their gut that help break down food and in the process the bacteria release metabolic byproducts as gas. The bacteria inside dragons released hydrogen that was channeled into the two storage sacs, the dragon's "flight bladders". Along with a super-light skeleton the gas filled flight bladders were the key to the dragon's flight capability.
Another evolutionary adaptation was the dragon ability to "breath fire". The inner surface of a dragon's mouth was almost armor plated. At the back of throat there was a fleshy valve. This flap was similar to the false palate found in crocodile throats used to stop their lungs from flooding when grabbing prey under water. Dragons used the false palate to prevent a back draft of fire from scorching their lungs.
Dragons had small glands in the back of their throats. These throat glands would squeeze out a deadly combination of chemicals which when heated by the dragon's own body heat and mixed with oxygen combusts into flame.
Dragons became naturally attracted to reflective objects because they like certain birds, regard shiny objects with power, thus leading to myths involving dragons sleeping in piles of gold.