This Mind-Blowing "Game Of Thrones" Theory Might Explain Hodor
There could be a lot more to Hodor than you think. Minor spoilers from Season 6, Episode 2!
Ever since Season 1, Hodor on Game of Thrones has been capable of saying only one thing.

That changed this week, though, when Bran Stark saw a vision of the past which included a young Hodor...then named Wylis!

Even more surprising was how eloquent and talkative he was. But what happened to turn him into the Hodor we know and love today?

Two redditors, u/TazoGreenTea and u/gbinasia, think that Hodor might have been a warg, just like Bran.

"I'm thinking that Wylis went far beyond the limits of warging which may have mentally damaged him and turned him into Hodor," theorizes TazoGreenTea. "I remember Jojen Reed warning Bran not to warg for too long or else he'll be trapped and forget he's human."
The theory goes even deeper than that: Since Hodor was a stable boy, it's likely that he would have chosen to warg into his favorite animal — a horse.

As we saw in the episode, Lyanna is very kind to Hodor and is also an accomplished rider. It's possible that Hodor could have warged into Lyanna's horse in order to spend more time with her and, as gbinasia theorizes, might have been in control of the horse when it was later slain, potentially causing him brain damage. Or he could have simply spent too much time warging and ended up taking on horselike characteristics.
This could, in part, explain why Hodor is so comfortable carrying Bran on his back using a saddle, and why he's afraid of thunder (as horses often are).

It could also explain why Bran is able to warg into him: If he actually has the mind of an animal, it should be pretty much the same for Bran as warging into other animals.
The only question left would be why Hodor only says "Hodor." But what if that's the name of Lyanna's horse?????

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vote votesIt's so crazy it just might be true!
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vote votesDunno. I'm going to need a little more proof.
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vote votesNah, he probably just got injured or something.
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vote votesI have a different theory, which I will tell everyone in the comments.