My 15 y/o has arthritis, and I put a pure MSM powder into her feed everyday, and it works wonders. It's about $30 a jar, but well worth it. Also, she's on a series of shots called Legend that my Veterinarian recommended - they run about $85 a shot, and she has to have one every few months. But, these two things help her immensely and she's got just as much get up and go as ever. :] Just make sure you keep up with the Legend if you decide to do it, because if you forget to get it done, she'll get sore again.
But,I agree, talk to your vet and find out what's best for your horse in particular. :]