I think it is human nature to combine all that is primeval and dangerous into a super beast. Most seem to be mash-ups of the scariest critters around. Big predators who can eat you; flying animals that can attack from the sky; and combined with some sort of reptile. Add the ability to make fire, and you have something to scare the folks with. Reptiles are particularly scary. Ever see a cuddly baby gator or a Cute Komodo dragon? Nope. They are hatched ready to hunt and kill. Combine that with dinosaur fossils which may have been seen, and you know there have been huge monsters from way back. In fact, ground dragon (dinosaur) bones were sold as medicine in Chinese apothecary shops until fairly recently.

Quite a time frame: The Early Modern age runs from about 1500-1800. The Dark Ages (sometimes called "Early Medieval") can range from 400 to about 1000 AD. Most of the surviving Medieval tapestries I am aware of, come from the late 1400s-early 1500s, and show unicorns as clearly equine in appearance --although some show goat-like beards and cloven hooves. Could you link to some of the goat/unicorn images?

Seriously, all the evidence of Neanderthal behavior indicates they were able to communicate. When I was in college, many years ago, the hypothesis that Neanderthals could not speak was based on their lack of a hyoid bone. Then someone found one. I suspect much of the question of Neanderthal language is a result of the 19th century progression model of evolution. Pre Homo Sapiens were all mute brutes, and only Humans were capable of language and complex thinking. That attitude has been mostly discredited, starting with a chimpanzee named Washoe. https://www.friendsofwashoe.org/meet/washoe.html

Orgilde onryne, yfel m¯aelan unornlic