My 9 year old ottb gets monthly Adequan injections. I've definitely seen a difference in the creaky hocks; he is reluctant to "sit down" and jump round. I had his hocks injected 2 years ago, and started him on the Adequan in lieu of the injections last year, and to me he feels very similar. Without anything, he is VERY swappy behind, and gets very up and down in his trot. With the Adequan, he is much more willing to stretch and reach.
Another vote for PENTOSAN! Joint supplements are hard though, every horse will react differently to each. GOod luck!
I had read some research saying you put your horse on joint supplements while still young, before they have problems/damage. I purchased a 7 y/o 1/2 Arab who has hind legs that are quite straight. He was a trail horse 'till then, so little stress. He was put on joint supplements right away. We competed for 10 solid yrs, making it to prelim. eventing. At 18, his hock x-rays were still pretty much clean, despite his conformation. At 24, he's still comfortable, if a bit stiff for jumping.
I've used Cosequin before as they are suppose to be the best about research. But now I use GLC. They have studies proving the absorbency of their product too.
None of my horses have ever been injected. If I have a problem flaring up, they go on MSM, which is very inexpensive.
No 'proof' that joint supplements work, but enough anecdotal evidence to make me a believer
I have had excellent results with Recovery EQ on an older TB/Percheron mare who has some arthritis in her shoulders. Made a significant difference in her comfort level (especially going down hills) and she was noticeably less comfortable when she was accidentally taken off it. It is not cheap, but it works (has kept the mare able to still be ridden instead of retired) and it is an oral supplement, so no injections. However, I wouldn't use it (or any other joint supplement) as a "preventative" as I haven't seen any scientific, or even solid anecdotal, evidence of efficacy whereas I have seen the right oral supplements definitely improve joint pain/arthritis issues in both horses and dogs.