This may be a long shot but could it be a nerve sensitivity? My horse was diagnosed and treated with lyme once before with the doxycycline. He showed the same symptoms again about 2 years later so I went a different, more natural route with treating it. I didn’t do a lyme test to confirm if he was positive for it but the symptoms went away. I found this product here https://effectivepetwellness.com/product/equine-lyme-disease-standard-kit/ and it includes a “nerve regeneration” within the kit. Just figured I’d throw that out there for you!