Aggressive/dominant behavior towards other horses
I have a huge problem. We had this really nice chestnut standard breed gelding for around 5months now who was a rescue case. 4 times a week we let him loose in a large field together with other horses which he absolutely adored. He is a big grumpy in general but very kind hearted and gets on well with humans and horses alike.
Since the past 2-3 weeks he suddenly started attacking one of the other horses and really goes for an attack. Lowering his head, ears back, teeth showing and then charges the other horse and bites viciously and gives chase if necessary. The other horse is not provoking my horse to attack and is terrified of my horse. Even if the other horse runs away and stays in a corner our chestnut suddenly approaches the other horse and charges at him.
We kept those 2 horses apart to avoid any injuries but now he started showing the same behavior towards another horse in the group and drew blood.
As soon as we separate them our chestnut is fine and even with his neighbor horses in his stable he never showed this kind of behavior.
We are really upset and cannot understand his sudden change in behavior when he is let loose especially since he was a horse that got on well with any horse.
Can anybody suggest or advice how to correct this behavior please?
There's no easy way out