Griffin: a lion with eagle head and wings
Medusa: a woman with snakes for hair which can turn people to stone
Beholder: giant eye which can float
This is even true of people like H P Lovecraft, who is often set forth as one of the most imaginative and inventive fantasy writers of the past 100 years: in the end, for all that Call of Cthulhu is a spooky story which admirably communicates the Lovecraftian ideal of inscrutable and never-ending indifferent evil, Cthulhu is merely a humanoid dragon with an octopus head. He is a "this with a this", or, if you are being charitable, a "this crossed with this with a this".
I've been sitting at my desk trying my damnedest to think of monsters that are truly novel, without antecedents - not composed of amalgamations or inspirations. Failing that, because of my human limitations, I changed tack and tried to think of existing monsters that have been created by fantasy authors or RPG bestiarists that I would call "new" - that were not based on the "this with a this" paradigm or regurgitation of folklore. I couldn't really do it; the only arguable, and honourable, exception I could think of was Geiger's Alien.
Why is this so? Some limit imposed by our evolution, which means that setting aside brain cells for pure speculation is a waste of resources? An immutable law of the universe which states that it is not possible to imagine a thing which is not based on prior experience in some way? That we are capable only of being what we are remains our unforgivable sin; our imaginations are shackled by our nature, it seems.