My 10 yo mare was dx w arthritis last summer. The vet I chose to work with is a leg specialist and he suggested monthly intramuscular injections of Adequan. No known side effects into liver etc. You can review it online. It does need to be done on a specific schedule and it costs approx $50 month. My mare appears to be doing fine with it…yet she has a pretty easy life and I don’t ask much of her.
I’ve talked to others that LOVE Adequan, indicating that it saved their horses life, etc.
You might also investigate Apple Cider Vinegar for arthritic conditions. It is something that I want to start experimenting with my mare once I can find dosage information.
Silver Lining Herbs has a kidney formula that they suggested for arthritis, indicating that in most arthritic conditions, the kidneys are suffering. I had my local vet do accupuncture for my mare when last summer when this all showed up. Her topline was pretty touchy and the accupuncture helped immensly! Vet administering it said it was all kidney reflexes that were firing off.
Good luck and may you and your mare have peace.