The only thing that might slow down or regress arthritis is IRAP therapy. It's expensive and it's only successful on certain types of arthritis; which is a question for the vet.
If a local vet is not set up to do the treatments, the horse has to be taken to a clinic or university that is. IRAP is a procedure that requires a sterile environment. IRAP II Therapy
In terms or oral supplements, I am a huge fan of Hylarin/Boswelia made by Med-Vet Pharmaceutical and sold thru United Vet Equine. I think Valley Vet might also sell it.
It is 99% pure, so it doesn't take much to be effective. This is all I feed to Duke (the Birthday Boy:), who has hock/ankle arthritis and he does really well. By this time, he is on the maximum allowable daily dose and it cannot be increased. His particular issues are not a candidate for IRAP which, my vet is a leg specialist and is set up to do - thankfully. United Vet Equine - horse supplies, equine supplies, horse nutrition, horse supplements, horse vaccines
Then there's that wahzoo clay poultice, Sore-No-More. It's an all herbal product using Arnica as the main herb. It is a cold poultice and does a fantastic job tightening the legs. I know it works because I slather it on my hands before I start trimming the horses.
While not really intended for arthritis, I slather it from the knees, down around the ankles on my horse with the torn ligaments because it does draw inflammation out.
I then wrap with vet wrap to keep the clay on the legs, then quilts, then standing wraps.
Sore-No-More also makes a liquid but I don't think it's as efficient for the more serious issues. Sore No More Cooling Clay Poultice Equilite (Health Care - Liniments)
That, in turn, means going thru a lot of vet wrap. I found that at 99 cents/roll at Schneider's Tack.
I bought six cases because, for 2-1/2 months, I was re-poulticing and wrapping Joker twice daily. That's two rolls of vet wrap for two legs every day Dura-Tech® Vet Flex Bandage in Wraps / Bandages at Schneider Saddlery
So there's some different things to consider, even though some are not totally dedicated to arthritis and are not preventatives. They still will help to draw inflammation/soreness out