Well, dragons were flying lizards that lived in dry hot climates and would swoop down to eat smaller prey. In the process, they would swallow stones. The stones would help digestion by grinding the dead carcass inside its stomach. Digestion was slower like snakes so over a period of time, gasses of methane would form. Since the bones and stones don’t digest, the dragon would occasionally puke up the stones and bones. In the process, the stones would hit against each other creating a spark that would ignite the methane gas and the dragon would burp out fire, spewing out the bones and stones in a fiery flash.
At times when these dragons would get into fights with other dinosaurs or prehistoric man, the jostling around would cause the stones in its stomach to hit against each other, creating a spark igniting the methane and the dragon would spew out fire.
And let’s not forget that mankind only came into existence 20 million years ago, while the dinos had all either died out or had evolved into birds 65-70 million years ago. The two never co-existed. Ever. It’s established scientific fact. Dragons are fairy story creatures, nothing more and nothing less, never actually existing at all.
Depending on how you define ‘mankind’, it actually arose between 1.5 million to 200 thousand years ago. I have no idea where you got this 20 million years figure from…
I knew they were before Neandertals. But! Regardless of that, I was wrong. I admit it. Dinos are more my line, anyway. I don’t mess around with the whole Cenozoic thing. Not my area of expertise.
Incidentally, that’s one of the very few cinematic depictions of dragons (along with Smaug) that actually make some taxonomic sense. Any vertebrate with six limbs is a taxonomic impossibility, at least in this biosphere.
The flammable gas it contains is likely less dense than the general atmospheric air and so this would help it fly more easily by making it more bouyant.
This is the same technique scuba divers use by inflating or deflating their vests.
Now if the dragon farts, it will have less flammable gas helping lift it up and won’t be able to fly as high.