Hi…just read your post about your horses behavior change. His symptoms SCREAM ulcer! I went through the same thing. After months of confusion and misery, at last someone suggest I treat for an ulcer.
I used quality aloe vera leaf juice ($20-$30/gallon). I gave him 4 oz twice daily with his grain…and had my horse back in no time. It was a miracle and I give him aloe juice once daily now as maintenance forever! I bought Pharm-Aloe brand at Whole Foods. There are other good brands out there. Just make sure it’s from a health food store and costs at least at much as noted above.
My horse had sudden behavior change after a move as well (can be very stressful for them). He got so spooky I was scared to be around him and didn’t trust him with anyone including me. Was worst time in my life! But I knew good horses don’t just go bad for no reason. Good luck to you!