My horses started with a slight limp and vet #1 squeezed, pinched , felt, poked/scraped with hoof knife and had no answer. Vet #2 (excellent equine vet) looked , felt, squeezed, x-rayed, topography scoped the whole horse and came up with the problem being a soft tissue damage in shoulder. Went home with medication. Limp got worse and the total shoulder muscle atrophied. It did not appear that horse was in pain and would run with the other horses with her awkward gait.
Subsequently ran across what appeared to be what happened since it mirrored my horses symptoms. A very little known problem called Sweeney, when the nerve endings in the shoulders are severely damaged. If known early, can possibly be fixed. In my instance, that morning my fence was down and a corner post broken. Assumed she had been kicked. Sweeney can also result from feet being caught (have page fence and it does happen) and we all know how a horse will struggle and throw itself to get loose, thereby causing severe damage to the nerve endings.
This was a more common problem in the old days when horses worked long, hard days with ILL FITTING collars, whereby they damaged all the nerve endings.
Another cause of lameness that few , if any, of todays vets will pick up on.
