If there is a common theme among the ancient people of the time
I should have specified:If the content of the texts is already known and in question
, we cannot turn to the texts for further substantiation, just like we can't find out if Herodot was right about some king by reading more Herodot.
People do share the same basic brain architecture. You can find commonalities in mythology just as you can find them in art, architecture, clothes design, and perhaps most interestingly, language. Common themes are just that, common - not therefore true.
To find out if monsters are a shared fiction or a shared observation, looking at the texts harder will do no good whatsoever. What you'd need to do is find other old texts for crossreferencing, and even that takes a way back seat to trying to dig up some fresh monster bones. In crossreferencing, you'd ideally find two fully independent sources describing the exact same thing in some detail. I'm unaware of any striking similarities. Monsters and dragons look different and behave differently in myths all over the world.
Thus my position: since the texts are lacking as evidence, we need to look elsewhere.