Thanks @gottatrot . Couldn't agree more. Thus the actions I've taken to attempt to gradually alleviate this condition as early as I could, in conjunction with professional DVM (and farrier) care. Your words and advice are well heeded.
@gottatrot , Just gotta tell you...
I've done autologous bone marrow transplants and PRP (for myself) 6 times. I'm a believer. I will do that for my filly if that's what she needs. But I want to avoid the NSAID route, and others, and pursue natural solutions - such as proper trimming - before addressing soft tissue injury(s) that she may have suffered as a consequence of her poor hoof condition when I got her.
I was reading that certain lines of TWHs can be prone to joint instability in the hocks that is called "wringing." This is probably what we were seeing in your videos where your horse's legs were bowing outward. This is different from twisting the hind leg after the hoof lands, which can be seen on horses with cow hocked conformation and/or long breakovers in some horses.
It is considered a fault by most since it adds considerably to the wear and tear on the joints over time, so you'd expect arthritis young in a horse like this.
You did mention that it has improved with better hoof trimming and exercise. To me this says you are on the right track, and I was reading that in particular strengthening the inner quad muscles with lifting exercises and hill work will create more stability so the horse will have less movement through those joints and less long term problems.
Perhaps in your experience EPM and DSLD are "out there" diagnoses, similar to the adage of hunting for zebras. However, you might keep these things in the back of your mind if you plan to have many horses in the future, because I've personally known two horses diagnosed with DSLD, and EPM is quite common also, with a few members on this forum having dealt with it. I'll agree that anemia or femur fractures would be extremely unlikely, but horses do fall and get pelvic fractures, and go for years without having them diagnosed. One potential clue is if the horse does not lift the tail.
I'm glad you posted here because it has been a learning opportunity, and perhaps you can post updates. However, I'll let you know that Big Lick horses/breeding are a very small niche and happening across someone with that type of experience would be a rare happening on a forum, unless you went to a TWH specific forum. The fact that people here with many years of horse experience did not recognize an issue relating to Big Lick breeding, and instead tried to assist with thoughts regarding other issues more commonly seen with horses is just because your horse is a rare individual. In my area of the country we have many gaited horses and many TWH, but very few bred for show (thankfully). I am used to seeing them flat shod and moving smoothly and strongly across the landscape.
Now I feel like I'm developing an eye for the faulty hocks. In this video at around the 2:30 mark the horse is walking away and you can see his hocks wobble.