Is Ser Robert Strong the reanimated corpse of Ser Gregor Clegane ('The Mountain')?

It is believed by many readers that Ser Robert Strong is the reanimated corpse of Ser Gregor Clegane, a kind of Frankenstein's Monster. The circumstances of Ser Gregor's death, Ser Robert's introduction by Qyburn and Ser Robert's physical characteristics strongly suggest this is true.

After Gregor's fight with Oberyn Martell, Qyburn (a former maester, who was stripped of his chain for unethical experiments he performed on living humans) asks Cersei Lannister to allow him to experiment on the dying knight. He convinces her and she grants permission to experiment on Ser Gregor's body, among other prisoners of the Red Keep. It is believed that Qyburn could have created a kind of Frankenstein's Monster and reanimated the corpse of Gregor into Ser Robert Strong. Qyburn says he is making an "undefeatable champion" for Cersei Lannister. Though none of his work is shown or described in detail, it's known he commissions a very large suit of armour for an unknown purpose. This armour is said to be far too heavy for a man to wear, let alone fight in. What other man in the Seven Kingdoms required a suit of armour as big as that? It is stated that Ser Gregor wears the thickest and heaviest plate armour in the Seven Kingdoms. And that no ordinary man would be able to move in it.

Furthermore, Mace Tyrell and Randyll Tarly both have doubts about Ser Robert Strong's identity. Meryn Trant claims to have never seen him eat or drink, and Ser Bors Blount says he's never seen him use a privy. It is never said what happens to Ser Gregor's body other than his head was sent to Doran Martell. Doran remarks that the skull he receives is certainly large enough to have belonged to Ser Gregor. Ser Robert apparently never removes his helm, so there is no conclusive proof that he actually has a head. Strong is said to never speak. Qyburn claims that Robert has made a vow of silence. But this sounds like a cover story.

While Bran is in a coma from his fall in A Game of Thrones, he has a dream while flying with the three eyed crow: "There were shadows all around them. One Shadow was dark as ash, with the terrible face of a hound. Another was armoured like the sun, golden and beautiful. Over them loomed a Giant in armour made of stone, but when he opened his visor, there was nothing inside but darkness and thick black blood." The first shadow is likely Sandor Clegane ('The Hound') as he wears a hound shaped helm. The second shadow is likely Prince Oberyn Martell as he wears armour of bronze discs, which he used to his advantage to blind Gregor in their fight. Also the sun is the sigil of House Martell. The Stone giant is likely Ser Gregor Clegane as he is referred to as "The Mountain" (which are of stone) and his helm is adorned with a stone fist. Moreover, the poison Prince Oberyn uses on Ser Gregor, is noted by Pycelle and Qyburn to have turned Gregor's blood black. Lastly, the stone giant is headless and Ser Robert Strong is possibly headless as well, linking to the fact he never speaks, never eats or drinks and has never removed his helm. Also, that Ser Gregor's head has been sent to Doran Martell. It is also worth noting that some people believe that Ser Robert does still have his head, but it may be disfigured due to Qyburn’s experiments, and that the head sent to Doran Martell is the head of one of the many dwarves being presented to Cersei when she puts an offer of Lordship for Tyrion’s head.

The chances that Ser Robert Strong is the reanimated corpse of Ser Gregor Clegane are high. It does seem very likely and it is very plausible.