The fact that Hodor was once able to say words other than "Hodor" wasn't the most discussed plot twist on the latest episode of Game of Thrones, but it was still intriguing as hell. Why would Hodor go from a kindhearted stable boy named Wylis to a severely impaired, gigantic man who mostly just carries Bran around and says "Hodor" with varying degrees of pathos? One fan thinks he knows why: Hodor was once a warg, just like Bran.
Writes user TazoGreenTea on Reddit:
Could Hodor possibly have been a Warg when he was Wylis?
Sorry if this has been mentioned before but I just thought about it this morning as a possible explanation as to what turned Wylis into Hodor. Given that Hodor could speak as a boy, it might be possible that he was once a warg or possibly had the greensight.
The three-eyed Raven once said that he's been following Bran, Meera and Hodor all their lives. Three-Eyed Raven link It's very possible that Wylis had similar visions that Bran had of the three-eyed raven. Wylis may have been more attuned to his abilities especially if his Old Nan told him stories and shared more knowledge about Warging. I'm thinking that Wylis went far beyond the limits of warging which may have mentally damaged him and turned him into Hodor. I remember Jojen Reed warning Bran not to warg for too long or else he'll be trapped and forget he's human. Jojen Bran link
Also, it would make sense that Hodor warged and identified more as a horse, since he was a stable boy. He's also afraid of thunder like horses Hodor thunder link and has carried Bran on his back using a saddle. Hodor carrying Bran pic
Hopefully an explanation is given in the later episodes as to what happened to Wylis.