If a large horse (1300 lb) is tied with a chain (e.g. something that will not break from the force) to its halter (again, assuming it will not break) to an overhead beam in a barn and the horse cannot effectively use its front feet when pulling back (i.e. its head is tied high enough it can't lower its shoulders and brace with its front feet, but it can support its own weight on its front feet), how much force will it or can it exhert on the overhead beam?

If the horse is able to pivot or twists/turns how much force can be exherted?

Not knowing much about horse physiology... are you saying that the horse has no ability to exert force against the ground, forward or backwards? Since his rear feet are free does that mean that he can push forward? If so, then the answer depends on how strong his rear legs are. But if not - then it seems the only force he can exert is his own weight - by lifting both front legs he hangs from the chain, and depending on the position of his center of gravity relative to where the chain is tied to the halter that would mean up to 1300 pound of force is put into the chain and the overhead beam.