I did a thread about arthritis a few months ago and it should be floating around somewhere, look it up, people threw in loads of suggestions.
Basically your on the right track with apple cider vinegar, for my 16.2h horse, I had him on one cup a day, 1/2 am 1/2 pm and it worked well for him.
There is also a product, natural sulphur, called MSM (methylsulfonylmethane) which has had remarkable success in humans, dogs, cats and horses. The powder can be mixed in feed, however as some horses can be finiky, I found it helped to core out an apple, stick it inside the cored area, and then plug both ends. Horses would wolf it down without a second thought.
Cheers! Aly