NERD ALERT NERD ALERT
Dragons are cool okay don't even pretend to deny it
But the best part about dragons is they could totally exist. Barring of course the fact that they'd have to eat all the time and barely have enough air to breathe (more on that in like 4.6 seconds), we've seen (read: dug up) dragons who have existed in the past.
Do you remember Petrie from The Land before Time? He was a pterosaur, a flying dinosaur with a wingspan up to 7.5 meters (which is probably like as tall as your house). Now, since the plant:animal ratio has changed a lot since they existed, the oxygen:carbon dioxide ratio must have also changed (because we spit out CO2 and plants spit out oxygen). Pterosaurs could have lived in a time with a lot more oxygen, allowing them to do a lot more physical activity (kinda how its harder to breathe super high up in the mountains).
But one thing that hasn't changed is Earth's gravity. The force required for a constant mass to hold itself above the surface of Earth shouldn't have changed much, and so if we strapped an oxygen tank to a pterosaur today, he should have no trouble using physics to keep himself afloat.