I used to use Pentaflex HA for my German Shepherd. It is made for both horses and dogs. While I don't know the effectiveness for horses as I never had to use it for my horses, my GSD made it to 17 years old with NO hip or joint problems. She was out hiking for miles till the day she died (she died due to other non-joint related problems). It is reasonably priced, although it can be difficult to find sometimes. It is powder form, and supposedly apple flavored (my GSD had no issues eating it). I gave my GSD the "horse dose" and one bucket lasted her several months (although admittedly I would forget a few days here and there and boy did I notice a difference in her movement when I missed days).
You can find it on Drs. Foster & Smith, Amazon, and I think Valley Vet Supply. It usually runs around $89 a bucket.