I can't read it, so Im going to assume what he is saying, from a family of equestrians point of view.
They tied the horse to the pole to keep it outside of the stall while they cleaned it, something spooked it and since its fairly young, it started kicking and bucking, then it ran into some stuff and it broke its leg. A broken leg on a horse is not fixable so they had to put it down.
Lesson Learned : Don't tie horses to poles. You probably had the rope too tight and it was uncomfortable
Hey dumbass, horses are tied to poles all the time, thats not a lesson learned. Not to mention they wear a halter so you don't tie ropes on there body parts.