I just found out that a horse we had was more than likely hypp positive. He became extremely dangerous while handling on the ground. He would start shaking and then all of a sudden, with no warning, start flying backwards, stumbling, falling, etc. He went through high tensile fencing on more than one occasion and almost trampled family members. He couldn't help it at all. After the episodes were over, he'd go back to being a normal horse. It was a sad day, but he was euthanized, for his own well being and the safety of family members. He just couldn't be trusted and there were NO warning signs, it just happened. Other than that, he was the sweetest horse in the world and rode like a champ. If that had happened with someone on him, they would have been seriously injured or even killed.