|Theories may be removed if ...|
It was believed by Kevan Lannister and members of the Small Council 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.
- Qyburn lost his Maester's chain for conducting unethical experiments on humans.
- It is mentioned at Harrenhal by Gendry that Qyburn is rumored to have knowledge of black magic and necromancy. 
- It is never said what happens to Gregor's body, except that his head is sent to Doran Martell.
- 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 armor for an unknown purpose.
- Qyburn convinces Cersei to allow him to experiment on the dying Ser Gregor Clegane, among other prisoners of the Red Keep.
- Mace Tyrell and Randyll Tarly have doubts about Ser Robert Strong's identity. Meryn Trant claims to never see him eat or drink and Boros Blount says he's never seen him use a privy.
- Strong is said to never speak. Qyburn claims that Ser Robert made a vow of silence, which sounds like a cover story.
- While the head of Ser Gregor is supposed to have been sent to Dorne, and Doran Martell 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.
- Ser Gregor championed for Cersei Lannister before Ser Robert. She may well trust in Ser Robert Strong simply because he is physically impressive, but it is remarkable that she stakes her life on someone that she barely knows.
"There were shadows all around them. One Shadow was dark as ash, with the terrible face of a hound. Another was armored like the sun, golden and beautiful. Over them loomed a Giant in armor made of stone, but when he opened his visor, there was nothing inside but darkness and thick black blood."
Evidence this relates to Robert Strong
- The first shadow is likely Sandor Clegane as he wears a hound-shaped helm.
- The second shadow is likely Prince Oberyn Martell as he wears armor of bronze discs. 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.
- The poison Prince Oberyn uses on Ser Gregor is noted by Pycelle and Qyburn to have turned Gregor's blood black. 
- The stone giant is headless and Ser Robert Strong is possibly headless as well.
- A Clash of Kings, Chapter 64, Arya X.
- A Dance with Dragons, Epilogue, page 949 (Hardcover edition).
- A Dance with Dragons, Chapter 65, Cersei II, page 859 (Hardcover edition).
- A Dance with Dragons, Chapter 38, The Watcher.
- A Game of Thrones, Chapter 17, Bran III.
- A Storm of Swords, Chapter 70, Tyrion X.
- A Storm of Swords, Chapter 72, Jaime IX.
- A Feast for Crows, Chapter 7, Cersei II.