There's significant evidence that arthritis is exacerbated, or even caused, by a chronically acidic cellular environment (low oxygen).
Try Glucosamine & MSM (methylsulfonylmethane).
Also try an alkaline diet, and alkaline supplements such as pinches of baking soda in your water and food (really), MMS (a Chlorine Dioxide solution), massively upping your dark greens and water intake, and cutting out acidic foods such as sugar, coffee, alcohol, etc.
I see no reason why the same principles wouldn't apply to your horse, too.
My horses and I have been on every anti-arthritis nutrieceutical known to mankind for years. And, they have all worked, to a point.