I would be getting a sample of that and pictures to your local vet asap. To me it looks like dehydrated tapeworm. If that's the case, you should be very concerned. Was this a single poop like that? Or are they coming out like that still? I would contact your vet.
Maybe some long grass stalk that couldn't be digested?
Tapeworms can be really long and roundworms can be a foot or more - it might be more than one worm tangled up in each other which is how they are in the horses system if there's a lot of them
Tapeworm on left, roundworm on right
I'm guessing a large roundworm! But totally wanting to know what the vet says!
When we were in our animal science lab a pig started pooping out roundworms cause she had recently been wormed. Looked somewhat similar but we only got to see them for a minute before they had been completely eaten up by the other pigs in the pen with her... They were killed by the dewormer so the instructor wasn't at all concerned but it definitely showed how easily they get around!
The vet took the sample on Friday to be checked out. I'll let you know. It's not very worm like at all. Since then all poops have been totally normal. It's bizarre. The vet thinks he ate something weird but isn't quite sure.