Athelas.
You are a medic, am I right? or a doctor should that be the case.
Apply some anatomy knowledge there and then try to reason how a being like a Centaur could exist? Can you imagine the sempiternal back aches? or how the heart would have coped with pumping blood through two bodies? I don't know much about siamese mutations not as much as I would like at least, but I think that when sharing a single vital organ it makes it incredibly difficult to perform things a Centaur could have done. Cyclops? yeah, definately I have seen modern day evidence of the mutation which causes the phenomena. However there are things which are definately old wives tales, by all means I believe that, however ruling out that all of what nowadays is confined to 'imagination' does not exist because a few mythological beings have no possible anatomical right to exist is going a bit too far. Otherwise we would have to rule out all the mythological creatures that are mentioned in the Bible too and Dinosaurs a while ago as well.
I have a great interest in hybridization and mutations, not the kind where little green men chop bits from you and put different ones, but our own and those existing in the animal kingdom based on that is why I feel there's a good possibility that things that don't exist anymore are in one way or another hybridized or mutated.
Take Dinosaurs and their evolution into birds for example.
Or another one which is one of my favourites.
I drew this theory pairing several disciplines to explain myself how metals described in books (fantasy usually) could have existed.
If we throw a line back to our primeval times of geological evolution we will find that most elements that are now separate into the many we know nowdays could have been part of one alone. Through the sieving and metamorfic function of geological dynamics we have got to the stage where these elements are in separate form or what we nowadays call 'pure elements'. But what if like nowadays stones like Ametrine (which have half the looks of a Citrine and half the looks of an Amethist), we have had a metal like which looked like silver but had high fusion point and ferrous capabilities like that of the steel? it would mean you could have hardened the stuff and still would have looked like a noble metal, if you wish you can draw the conclusion you wish on the metal I'm referring to.
I use that same one when trying to explain to myself the nature of the one ring rather than stick to Tolkien's argument of the 'evil nature of gold' kind of crap. And then I think, what if it had been gold in colour but with such a high fusion point due to other traces or elements that only working it somewhere where you can achieve extreme temperatures (e.g A Volcano) would have made it possible? You have such elements in this day and age which can only be worked (not smelted) on extremely hot blast furnances (way hotter than your usual home made kiln).
I could carry on, but I think this conveys what I wanted to say.
I'm loving this topic and certainly look forward to its development