The scream sounds like a loud roar of rage. It's very rare to hear a domesticated horse scream. It is only used during a fight between two horses, usually in the wild.
"I WILL KILL YOU"~ The horse roars at another horse, moving towards the other horse aggresively, striking out and/or biting.
"THE FIGHT IS ON!"~ Both horses move toward each other aggressively, striking, kicking and biting. This continues till one of the horses back down.
"YOU ARE THE BOSS" or "YOU WIN"~ The horse backs down, with it's tail set low, running away from the other horses threat.