It sounds like normal herd behavior going on with your bunch. The gelding is trying to make the mares submiss so that he has total control over them. Probably they are more bonded to each other than to him and that is driving him crazy. Stallions in the wild will herd his mares until they become submissive and obedient.
It is also possible that your gelding is just a bully. A good herd leader will establish his/her authority and then leave the other horses alone, only discipline them if they are disrespectful or disobedient. However, some horses are just bullies and will pick on the other horses in the herd relentlessly. Perhaps it is coincidence but I have known quite a few appy geldings that were bullies. By now, the herd hierarchy should have straightened out and he should be treating the mares better. If not, he may be a relentless bully and may need to be separated from the girls.
If his aggressive behavior continues, you may want to consider the use of an electric shock collar. It is only used in cases where it is in the horse's best interested to eliminate the unacceptable behavior, which in this case is aggressiveness. People have had remarkable success with aggressive horses in very short order. The shock collar is similar to what they use on dogs, but with a much lower level of stimulation (at the lowest level, a human cannot feel anything). Basically you put the collar on the horse and from a distance, you give him a shock every time he acts aggressively. Generally, in one or two session the horse is cured.
Good luck with your horse. I hope they have settled by now.
Julie Goodnight, Clinician and Trainer
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