3. Ser Robert Strong-An Alias?
Now, this is the beginning of the spoilers for those not far into the books or only watching the TV series. This reveals the theory by just looking at the picture. As we know, there is going to be a jaw-dropping fight later in the show between Gregor Clegane and Oberyn Martell. I am very much looking forward to this showdown and while the outcome is known, the fight itself will be legendary. Anyway, both entrants die from the fight, with Oberyn receiving a face crushing grab and Gregor dying from manticore poison. As it stands, there aren't many people in Westeros who are almost eight feet tall, weigh over 30 stone and can wield a great-sword in one hand. So, when a mysterious figure of eight feet and arms thick as tree trunks poofs into existence after The Mountain has died, there is something fishy about. Maester Qyburn was given permission by Cersei to experiment on Gregor while he was dying and Qyburn is an enthusiast of the dark arts and necromancy. With members of the Kingsguard reporting that he doesn't eat, sleep or use the privy, it is safe to assume that this monster is Gregor's reanimated corpse. Also, the giant just happens to have sworn a vow of silence, rendering him, coincidentally, unable to talk. Gregor's body whereabouts is also unknown, apart from his skull being sent to Doran but he has never removed his helm, so there is no proof he has a head! While a walking, fighting, eight-foot, headless reanimated corpse seems a little far fetched, stranger things have happened such as Melisandre giving birth to a shadow demon and Bran becoming a weir-wood tree. I believe this to be true, especially because it links into the next theory I have lined up.