WARNING SPOILER ALERT!!!
without bashing religion too much, here goes...
the fact it was written long time ago, before believing that the world was round and thinking they'd fall off if they went to far... let's assume they're ignorance. describing, re-enacting things, relating experiences etc, tends to lead to exaggeration so talk of dinosaurs and fire breathing is simply a venomous bite... they cant explain why their skin bubbling up, burning pain, melting/deteriorating flesh so they come up with firebreathing... fear with paralysis, was the creature so scary looking (then again back in the days serpents and reptiles were huge so that can be plausible), was most likely neurotoxins.