Weakness in the hind end can be a symptom of nerve damage in the spine, sometimes called wobbles. I had a horse who was diagnosed as a wobbler, the vet told me it could have been something he was born with, or it could have happened during a fall. Hard to tell which one, as he fell under the category of a wobbler, generally happens in males, and tall ones with long necks. Wobblers can be hard to diagnose, but some of the symptoms I experienced with my horse was he looked like he was drunk when he tried to walk, he didn't know where his feet were, and he was not a typical case in that his weakness was in the front end rather than the back end. He would trip a lot and fall down on his knees. He also would fall in the pasture while playing. The sad thing is that their minds and personality are not affected, so they will act like a healthy horse while their body falls apart around them. I hope you are able to rule this out for this horse, as it is usually a sad ending.