Most readers probably believe that Ser Robert Strong is what's left of Ser Gregor Clegane (sans noggin). I lean that way but, I wonder, how was it done and how is he controlled. We know that he has delved into the darkest of studies as a Maester to the point that the Citadel took away his chain however, it was never said that he dabbled in magic. Without a chain, it's more difficult to tell without him coming right out and saying so.
So, the question is: Did Qyburn use magic or alchemy to wake this giant monster? Which ever one it was, I believe we'll see he bit off more than he can chew (much like Frankenstein).