I'll try to be very straight to the point here:
- Both Duncan the Dall and Coldhands are described as tall characters. Both have dark eyes, as well. Most of Coldhands' physical descriptions focus on how dead he is, though, so there isn't much to go on here.
- Both share a great love for their peculiar mounts. Dunk always shows respect and puts great thought to every single one of his mounts. When Chestnut dies, he's clearly very, very saddened by it. And that's exactly how Coldhands reacts when he realizes the great elk he was mounting has to be sacrificed.
- Duncan is a big-time friend and ally of Brynden Rivers'. Who's now living inside the tree North of the wall, and who commands Coldhands.
- The only fraternal relationships Brynden had previously to his disappearance that we know of are with Aemon, Dunk, and Egg. We know Coldhands isn't Aemon.
- About Coldhands, Leaf says: "They killed him long ago". To me, that seems to indicate that he was dead a long, long way before Benjen, Will or Waymar Royce disappeared. It's ludicrous to think that child of the forest, who lives hundreds and hundreds of years, would describe a period as short as the two (?) years gap between AGoT and ADwD as a "long" time.
- The first time Dunk is mentioned by name in the main series of books is when Aemon tells Samwell about the time he was brought to the Wall alongside Brynden Rivers and, you know it, Duncan the Tall. They used that line of dialogue to introduce Brynden, who would eventually show up as a major character in ADwD... And also tease Coldhands' identity.
The potential 'holes' of this theory:
- Dunk supposedly dies alongside Egg and his son Duncan during the Tragedy of Summerhall. I use the word "supposedly", though, because this series is notable for its unrealiable narrators and for George's love of fake deaths. And old tale told me Dunk died there... But I'm not sure I believe it.
- Coldhands calls Samwell "brother" at one point. That seems to indicate he was once a man of the Night's Watch, which is, as far as we know, not an apt description for the Lord Commander of the Kingsguard Dunk. Still, given the possibility that he did not die at Summerhall (which I did pull out of my ass for now, yes), there may be a lot of missing pieces in his tale. We do know he knows his way to the Wall, at least, considering that he had already been there alongside Brynden.
It may be one of those crazy theories, yes... But I was really just left thinking after the idea came up. Why did George choose Dunk and Egg as protagonists for his prequel tales? Yes, they're interesting characters with eyes through which fans are able to know a time before the fall of the Targaryens. But things are never that simple in this series, are they? Those stories have already served as a way to set up Brynden's myth, after all...
Maybe the answer to one of this series' oldest mysteries is right in front of us. Just a random thought.
