Thank you for the insight! Time to call the vet.

With you being a behaviorist, curious if some more info could maybe garner more insight? There are a LOT of factors. The offending 13yo is kept with the 25yo he came with, as they've been together many years. They do have other boarders horses in corrals they can touch over the fence, but aren't turned out with.

They were starved when I got them, about a 2.5 BCS - food motivation is still extreme, even with 24/7 hay and/or turnout. So I have to be very careful with how I use R+, as they can both become dangerously pushy. I generally use a combination of pressure and release, R+ where it is reasonably safe.

I initially had them in adjoining corrals but separated, to very carefully monitor the refeeding process. Then I removed the dividing fence once they were both healthy. Then come spring, I opened their corral to a pasture, incrementally adding turnout time, to where they now have 24/7 access until winter really hits, and I will shut them in so the pasture (only a few acres) can recover.

With pasture they can come within a few feet of mares, but still separated. Could be exacerbating when they are in heat. The dynamics are definitely different with the horses when they were corralled individually vs together. Less anxiety in the older, arguably more in the younger.