I would agree that it is worms, but I would think that you would be able to tell a worm from a strand of hair. Maybe you could put on a pair of gloves and pull out one strand and see if it really is hair.
I agree that if he were housed with another horse it's possible that he just groomed the shit out of them and is now packed with hair, but I've known a lot of horses pastured with friends and never seen one pooping obscene amounts of hair. Based on what you've said it sounds like the other horse would be bald by now.

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