I know a horse with a very strange behavior. He was an abused horse when he was rescued. He was very aggressive and would attack anyone. When he was tied if more than 2 persons were in the same room with him he was very stressed and would bite the lead rope, a metal bar near him, anything. He still does this when you are grooming him or when he's nervous. He had a couple of months when he was a saint, he ended his aggressive behavior. After some time he started again to be as aggressive as he was.
Now only his trainer can work with him. Sometimes he is calm and very obedient and next day he could try to kill his trainer. He does this for over an year now. He is with the same trainer, under the same training and under the same methods as always. Can't understand why sometimes he is obedient and sweet and after a moment he tries to kill someone. You can be near him, grooming him as always, praising him etc and suddenly he freaks out, squeals and then attack you. After the incident he is back to his normal self as nothing happened.
He can be just an aggressive horse or he can have some mental problems.