In "Home," the second episode of the new season of Game of Thrones, fans were let in on a little secret: Hodor's real name is Wylis—and he used to have a much more extensive vocabulary. (In the books, the stable boy's name is Walder, but they likely changed it for the show so as not to get him confused with the universally hated Walder Frey.) This interesting reveal leads to a lot of new questions about Wylis's past and family, namely: Who is Wylis and why can he only say "Hodor"?
As might be expected, there are already some compelling theories on Reddit. One Redditor, TazoGreenTea, has a theory that Hodor is actually a horse trapped inside a man's body. Which may sound a little crazy, but stick with us.
As Old Nan explains to the Stark children in the book, Hodor got his nickname because it's the only word he can say. She's not really sure where he picked up the word; some people believe that he was kicked in the head, since being a stable boy means a lot of contact with horses. TazoGreenTea thinks that Wylis's connection to horses might be more mystical. Since he is also from the North, it's reasonable to suggest that Hodor—like Bran Stark—has similar warging abilities. And that it's possible that Wylis would warg into the horses and one day took it too far.
"I'm thinking that Wylis went far beyond the limits of warging, which may have mentally damaged him and turned him into Hodor," TazoGreenTea wrote. "I remember Jojen Reed warning Bran not to warg for too long or else he'll be trapped and forget he's human."
It's true that Jojen warned Bran about the dangers of warging, but that was more likely because Bran was gone so long that he didn't eat and his body was suffering. The Three-Eyed Raven also warned Bran about warging, telling him, "It is beautiful beneath the sea, but if you stay too long, you'll drown."
Given this warning, it's possible that if you're gone too long, you begin to forget who you are—and that Wylis stayed inside a horse too long and didn't come back fully human. Some evidence that would support this idea is that as Wylis, he could speak and was eager to fight, but as Hodor, he cannot speak and is afraid of violence. Further clues, as the Redditor points out, include Hodor carrying Bran around on a saddle and his crippling fear of thunder.
Another Redditor, gbinasia, built on this theory by bringing in Ned's sister, Lyanna. He explains:
As a stable boy, it would make sense that his choice would be to warg into a horse since, like dogs, they are docile and are accustomed to his presence. And if the (possibly) only girl treating him nicely was Lyanna, it would make complete sense that this is the horse he would warg into. White is a symbol of purity, innocence, and Wylis/Hodor is all that. He could warg into that horse so that he gets pet, brushed and generally receive[s] attention no girls would give him. Since Rhaegar and Lyanna would eventually run away from everyone, it also makes sense that they would both be on horses while doing so and Wylis/Hodor would have been witness to many events of that storyline.
The Redditor goes on to suggest that the horse he was inhabiting was possibly killed, and that Wylis and the horse mixed when reentering his body. As a human and horse combined, Hodor is both simple and skittish. This would explain how Bran was able to warg into Hodor's body, since humans are much harder to inhabit than animals. If that's the case, it also explains a lot about Hodor's gentle personality.