Some interesting ideas, but not really on topic Ceranko!
Dragons, eh? Sumigo is right that they are perhaps overused, in some respects. I think this is in part because they seem to represent the "ultimate" in natural power. If you take angels, demons and other semi-divine beings out of the equation, few creatures seem to rival the power of a dragon. Griffons, hippogriffs, pegasi, trolls, cyclops, hydra etc... Don't all of these seem to be lesser? So in that sense, I think dragons fit the bill.
I would argue that another element is their serpentine nature. Dragons are like snakes, scaly and inhuman. Snakes have long held a pre-eminent place in Western symbolism (ie the bible), and I think dragons are an extension of that. I can't really reason it out thoroughly, but dragons are also held to live a long time, like snakes that hibernate. In the context of epic fantasy, it's very convenient to be able to call up an irate dragon that perhaps no one knew existed. Other 'monsters' might have to live in a colony, catch prey and be obvious to the outside world, rather than being more elusive. Is that convincing?
Plus, unlike the other creatures I've mentioned, dragons are obvious. Everyone has an idea of what a dragon looks like, so an author only has to spin their own interpretation. By contrast, few people might readily be able to distinguish a Griffin from a Hippogriff

. I'm not sure I could either.