Question about joint supplements (kind of long)

That makes sense. I think it's more like what you said, since this is the first time he's ever been sore on that leg. He's barefoot, so it's not a shoe problem. Thanks for the advice. :)

Anebel-
I was thinking of something else when I herd injectible glucosamine I guess. I did some research on IM Adequan. When my vet suggested injections. It sounds like a great product, but the loading dose is pricey. I may start giving them in a few years if I ride my mare again. Her bigger problem is navicular though, the vet said we could do coffin bone injections once a year. Im not against it but it sounds somewhat invasive.

IM Adequan mostly acts to maintain and improve the condition of the cartilage in the joints.
A mare I used to own had some navicular and arthritis in her coffin joints and I found that the Adequan really helped her. It is something that you have to have a certain amount in their system, hence the loading dose and the fact that it must be injected every 30 days, no longer.
Coffin bone injections are also a good option as long as they are not being done more than twice a year. They really help, but are expensive and invasive. You have to make sure that you are not injecting with steroids, as this just covers up the issue and leads to faster joint degeneration. Injections with Legend into the joint are usually mixed with an antibiotic, and the injection site is thouroughly sterilized before and after the injection (usually with a betadine scrub solution). So if done properly, they are quite safe.
IV Legend is usually what people use to keep oldies sound and pasture puffs comfortable. There is no loading dose and no set length of time between injections - you just give it to them when they need it. Using a generic and a local vet can reduce the cost of these injections to around $75-100/horse including the vet trip.

Is this something the vet has to administer? I've given IV and IM myself before.

I've always acquired it through my vet and had it administered that way. You can administer it yourself, and then you can also buy in bulk (same with the Adequan) which is much cheaper. Some vets will sell you Legend and Adequan (or their generic counterparts) while others absolutely will not.