Here's a quick follow-up to my previous post:
What's the origin of the dragon motif? I don't know. But let's speculate. If you're a young-earther, you might say this reflects historical memory regarding pterodactyls. These were prediluvian creatures that became extinct. They undergo literary embellishment as fire-breathing dragons. Maybe a conflation of pterodactyls with lightning.
Here's another possibility: in their various depictions, dragons seem to be reptilian. Snake venom can have an intense burning sensation. It feels like putting your hand in fire. Perhaps, due to literary embellishment, this became the ancestor of fire-breathing dragons.