I’ve had 2 horses with ulcers that were both extremely spooky. With spooky behavior I always assume it to be ulcers until proven otherwise. Start with that first.
I’m still thinking pssm or pssm 2. My pssm type 2 mare cross canters behind whenever her muscles get tight.
I had a mare on lease that came in skinny… I put her on the best hay and a cup of oil in the morning and night. I thought she had ulcers so started treatment for that… I didn’t know what was wrong with her until she tied up one day just out in the pasture. Her muscle disease was something different than my other horse. The owner opted to pick up the mare so I never did find out what was wrong with her other then she was prone to tying up. She was a beautiful horse but clearly not well. I was pouring feed into her with minimal improvement in weight.
My type 2 pssm mare has a poor top line and poor musculature in her hind quarters. That seems pretty consistent regardless of how much feed she gets. However, she’s not particularly difficult to keep weight on and neither horse was reactive/spooky.
When she gets startled does she buck? Or buck at the canter? My pssm mare did a lot when she was started under saddle- it was a reaction to the muscle tightness. Fortunately she’s a good girl under saddle and never tried to get anyone off. She was just trying to say “I can’t do this!” As long as you listen to her, she tells you how she is feeling and you adjust the ride accordingly.
Muscle diseases can be very difficult to figure out as you would need a muscle biopsy and the genetic test for type 1 pssm.
I would also get her teeth checked if you haven’t done so already.
