Left-Hand Percherons has great advice. Just the caveat I want to add is that the established band of mares-solution *might* work for a horse who is aggressive, but doesn't really want to be alpha and who's horse-aggression hasn't become a completely ingrained habit.
The mare I have owned for over 11 years has horrendous equine social skills and really cannot be turned out safely with other horses. She's never quite grasped that one can be dominant and move another horse with fairly subtle body language -- rather, she charges other horses, chases them, and then whirls around and tries to double-barrel them. Sometimes she's been able to go out with one (extremely submissive) horse she likes and been okay, although she'll have days where she chases the horse around. You can't increase the herd number to two or more other horses, as she'll select one as "her" horse and be dangerously aggressive about driving the others away. Putting a horse like this with an established herd with a strong but fair alpha mare just makes her act even nuttier and more aggressive, as the alpha mare is unlikely to back down and run from her.
However, when she's being ridden or handled by people, she's a perfect angel. Provided something else doesn't provoke her by trying to kick or bite first, she wouldn't dare make a face or lift a leg. That's the best you can do, in my experience, with a horse-aggressive horse. Train them that they cannot behave that way when humans are around. Horse-aggression when you are in the way is about as serious a NO as it gets, just short of being aggressive towards you. My horse has always been sweetness and light towards people but when she threatened another horse while I was in the position to get caught in the cross-fire, I went batsh*t crazy at her, yelling, jumping up and down, chasing her into a corner of a stall, thwacking her with a lead rope, basically making her think she was going to die. She never did that again.
Last edited by thesilverspear; 02-10-2011 at 08:19 AM.