Some guy with a sword kills a big croc, which is a good feat! Yet each telling the croc is bigger, more fierce, especially as the alcohol flows.
His grandkids hear the story too.
Some ancient armies used long cloth bags as flags or banners, to impress the enemies or maybe to signal the position of themselves for the allies to see. They filled the bags with hot air and the bags rose to the sky as contemporary balloons.
Imagine how it looked from the enemy side: an army is approaching and long snake-like things are flying about the mass of people. And you can glimpse some fire at the end that is close to the ground. Could it be a creature that breaths fire? And occasionally these things detached and flew up to the sky. Perfect dragons.
I'm not sure about genetic memory, but human oral tradition is very old, and while I haven't heard about any evidence suggesting it predates modern humans, I wouldn't be surprised if there are stories that turns out to be as old as human language is.
https://commons.wikimedia.org/wiki/File:Pachycephalosaurus_w...
If you're in a pre-civilized, just-barely-scraping-by survival mode, you'd probably conclude: (1) this thing was a monster, and if it lived here I'm not safe. (2) This thing must have been killed by something worse, and if it's around I'm really not safe.
Imagine finding a 12 foot long skeleton in a cave, or just a skull of an impossibly large animal, of course it’s a dragon!