Is this theory a stretch? Absolutely. But is it still plausible? Sure, why not – we literally know almost nothing about Hodor. Nothing. So to say that he’s a warg who transferred his mind into a horse and then that horse was killed…eh. Why not? That’s just as likely as Hodor having the opposite of Tourettes (which doesn’t exist), and just as likely as him just being a stubborn ass who refuses to say anything but “Hodor” and has been trolling people all this time.
Luckily for us, it appears that with Hodor’s inclusion in Bran’s flashback this past Sunday we’ll finally find out exactly what he is and why he’s the way that he is this season. Personally, I’d rather they tell us where the fuck Benjen Stark is – Hodor isn’t going anywhere anytime soon. At least give us a hint about Benjen, huh?