how old is the horse? also, is it a front leg or a hind leg? a lot of times younger horses can get what's called a "locked stifle" but this occurs in the hind legs. What happens is the tendons in the stifle move and cause the patella to "lock" and when they unlock it it sounds awful (like the popping tree branch you described) but I would suggest having a vet come out. Let me know though, I'm a pre-vet student and am curious as to these kinds of lameness issues that occur. Thanks. and Good luck.