I have gotten the best results with liquid Corta-Flx. My 20 year old QH mare with arthritis showed significantly improved movement within 48 hours of being on the product. Another good choice would be Equinyl CM (pellets). Flax seed, with its abundant omega-3 fatty acid profile, also provides anti-inflammatory properties and may help with arthritis. Feed approx 1 oz per day (a heaping tablespoon) to a 250 lb mini.
There are many anti-inflammatory type products on the market, most of which contain glucosamine, chondroitin, MSM, hyaluronic acid, devil's claw, etc., or a combination of many of these ingredients -- all of which are considered non-prescription neutriceuticals. They should all be considered "medications", however, and given with due care. Remember that devil's claw should not be given to pregnant mares as it can cause abortion -- so be sure to read your product's label carefully and follow the manufacturer's directions in terms of dosage. If there are no instructions for ponies, I generally provide 1/4 of the full size horse dose unless instructed otherwise.
Robin C