Well, I believe Leviathan breathed fire simply because the Bible says so. This is also backed up by many reports of "fire-breathing dragons" reported by many various civilizations over the centuries. Just because people think it fantastic doesn't mean it isn't so.
Here is petroglyph of a fire breathing dragon:
Here is the caption that came with this photo:
On the right, one of the curious "dinosaur" petroglyphs near Middle Mesa at the Wypatki National Park, photographed by Chris Maier after a guided two day hike to the site. This particular petroglyph is called "Puff the Magic Dragon", and appears to be a depiction of a fire breathing dinosaur. It is not possible to date petroglyphs because the rocks from which they are made contain no organic materials.
"The ages of the petroglyphs on this panel and the ones we would later see are unknown. They are believed to be at least several hundred years old, but they may be as old as a thousand years or more."
There is no way to positively date this petroglyph, but it is thought to be at a minimum several hundred years old, well before dinosaurs were discovered.
It is also possible that these creatures spit a very caustic chemical. The venom of the spitting cobra is known to burn the skin. Very many creatures such as jellyfish have the ability to burn the skin. So perhaps the fire breathing dragons spit a very caustic venom that could burn a person to death. When you consider the numerous creatures that already have these abilities, it is really not so fantastic whatsoever.
The electic eel is known to produce an electric shock strong enough to stun a horse, so why is breathing fire so unusual?
