Recognise bullying for what it is
Every herd has a hierarchy, and a certain amount of ear-pinning, tail-swishing and being moved off a hay pile is normal horse politics. Bullying is different: it is targeted, constant and one-directional. The victim hovers at the edges of the group, drinks and grazes only when the others are settled, and starts appearing with bite marks on the neck and quarters, kicks landing on the ribs, and a coat that looks rougher week by week. He may hang at the gate at bringing-in time not from eagerness but because the gate is the one place the pressure stops. Weight sliding off despite good grass and hay is the sign owners most often miss, because the horse looks merely 'a bit poor' rather than persecuted.
Watch before you change anything
Spend a few whole feeding and turnout periods just observing who does what to whom. The aggressor is often not the horse you would nominate — sometimes it is a middle-ranking horse who bullies downward precisely because he gets pushed around himself, and sometimes the real trouble only happens at one pinch point: the gateway, the water trough, the shelter doorway. Note when the incidents happen and where. Ten minutes of watching will tell you more than a week of guessing, and it tells you whether you are dealing with one bully, a pair who hunt together, or a horse who is simply too low in this particular group's order to thrive in it.
Rebalance the resources
Most field bullying is resource defence, so change the geography. Put out at least one more pile of hay than there are horses, spread wider than a kick's reach, and never heap forage in a corner or against a fence where a low-ranking horse can be trapped. If the field has one trough, one gateway and one shelter, those are the three pressure points: widen gateways where you can, choose long troughs or a second water point, and accept that a single small shelter shared by a horse who will not enter it is no shelter at all. Bring the bullied horse in, or into a taped-off section, to eat any hard feed in peace, and let him finish before he goes back out.
Change the group if management is not enough
If the geography is fixed and the bullying continues, change the cast rather than escalating. Moving the aggressor out often settles a field faster than moving the victim, because removing the victim simply returns him to the same horse when he goes back. If the victim must move, pair him with one calm, friendly companion first, with fence-line contact with the old herd, and build his new group around that friendship. Some horses are genuinely happier in a pair than in a big herd, and there is no failure in accepting that. What does not work is hoping a determined bully will get bored; the pattern usually deepens.
When it stops being a behaviour problem
Bite wounds that break the skin, a horse coming in lame after a kick, weight that keeps dropping despite separate feeding, or a horse too frightened to lie down and rest properly all deserve a vet's involvement rather than more field management. It is also worth remembering that a horse who has suddenly become the aggressor may be defending himself against pain of his own, and that possibility belongs in the same conversation. When the notes on who bullies whom, where the extra piles go and how the quiet horse is eating are shared through KinsLe's Family Sharing, every family member who walks the field is looking out for the same things.