Mack has really chronic arthritis in... well just about everything. For the last three years I've been using Total Joint Care by Ramard with really great results. Within a week my horse moved better than he had in five years. (Granted he got kicked a month later, but hey. I'll take what I can get.) I started using it after my boss started using it on her jumpers and she was able to totally eliminate adequan. It's a bit pricey, about 60 dollars a month, but you can save a few dollars by buying it in bulk. You can get it online at Dover Saddlery.
A link: Dover Saddlery - Search Results for total joint care