During the Middle Ages, tales of struggle with dragons probably arose due to the presence of "horse eels" found *and hunted/killed) in the larger streams of the British Isles, and possibly elsewhere in western Europe. These critters are now rare, probably now (recently?) extinct, other than in Loch Ness.
In classical antiquity, dragon myths come from fossil forms remembered in Scythian folklore. The dino fossils were visible in the cliff face somewhere in Central Asia, I think even the location has been rediscovered in modern times. Adrienne Mayor's "The First Fossil Hunters" is the reference on that.
Dragon myths in other places with no solid folkloric connection with the two above do tend to be ancient, but refer to dragons in the sky, hence, likely large comets which spent periods visible in the sky, or large messy bolides coming down, breaking up, and appearing to be at least two entities struggling in battle.
...Bones helped to put ancient peoples in touch with the past and to vivify their mythology for them, but so did the recreation of these myths spatially. As it turns out, providing an area for reliving Roman myths was something not uncommon in antiquity or today...
This fact was particularly true for the private collections held by Roman emperors, which were often displayed in gardens and grottos. The so-called antrum Cyclopis (Atrium of the Cyclops) became a common feature installed in Roman villas in the imperial period. It was usually a watery grotto with sculptures of Polyphemus, the cyclops from Homer's Odyssey, and other scenes from Odysseus' travels...
Even into late antiquity and the early middle ages, the alleged bones of mythical creatures drew crowds. The emperor Constantine had a fascination with them. Saint Jerome states that the early 4th century ruler travelled to Antioch just to see the bones of a satyr that had been preserved in salt. The 6th century Byzantine historian Procopius notes that he stopped off in the Italian city of Benevento in order to see the 27-inch tusks of the Calydonian Boar famously battled by Greek heroes. As Mayor notes in her book, these were likely the tusks of woolly mammoths and not those of the mythic boar--despite what the signs at Benevento told visitors.Roman Emperors, Monster Bones, And The Early History Of Fossil Hunting | Sarah Bond | Forbes | June 29, 2016