I have seen that Apple Cider Vinegar can be given to horses to help keep their joints and muscles free of calcification and other health benefits. It works as well for them as it does for us, and I have just bought 5 liters of ACV with 'mother' from an Equine Store for around half the price the same quantity would have cost in the Health Store for my own personal use!
It may take a few months of treatment and you may also be able to use it topically too around the joint.
If you put 'Horses Cider Vinegar' in your search engine you will get quite a lot of info.