In situations where a horse is stressed from what they perceive as threatening, the horse will either go into fight or flight mode.
As a prey animal, it is a horses first response to take flight and run away from danger or perceived danger. In flight mode, a horse will only stop after putting distance between itself and the perceived threat, turn around and check whether the danger was real. In a herd, even if only one horse perceived the threat, the whole herd will take-off together.
Fighting is a last resort to defend itself. Ie. A horse would prefer to run away from a predator than risk fighting it. Typically, if a horse has no option to take flight and run away, will they start kicking, striking and biting.
Fight vs. flight is a survival response in situations where the horse feels threatened.