One of my boarder’s horses is typically an extremely easy going, quiet gelding. He’s a Canadian, so he’s a big guy. He was so good that when I was struggling to put my wiener of a two year old with a group, I chose him and a couple other dead quiet ones who all were predictably kind. For over a year that group was exceptionally tolerant of each other, not a single pinned ear, they would all smush together in the run-in or share from the same pile of hay. Then one day, this Canadian gelding turned on one of his herd members. He would chase him around constantly for no reason, even if the horse was on the other side of the paddock and not even near the others or the gate. It was horrible and we pulled the gelding that was getting chased out. I wondered if something had happened between them to weirdly set it off because it was so impossibly out of character. Another five months went by, and then this Canadian gelding suddenly turned on my young guy the past weekend. Chasing him into the fence, pinning him in the run in, trying to essentially kill him. Luckily people saw it happening and got them out. This time I pulled out the Canadian and put him in with the group of big dominant geldings, hoping that being with horses that stood up for themselves would dissuade his random aggression. But I also told the owner that if he showed any more signs of aggression that she would be given notice to leave. Note that this all happened in a huge grass field that they have been in for months. I have not changed his food, supplements, routine, group members, handling, turnout, etc. They have not been near any mares in heat. Has anyone ever seen this happen with a horse before? I can’t say enough about how much this horse is a saint on the daily with his group otherwise.
I have a mini gelding like this. I turned
him out for months with my other mini, then one day he charged him, turned, squated down and backed into/under him and double barreled him so hard the poor victim went flying up in the air and pancaked onto his side. It was vicious. The people that witnessed it thought he killed the other mini as did I. They talk about the incident to this day. So that turnout situation ended. He then proceeded to randomly do the same thing months later to a big pony who he adores. Now he gets turned out alone. I have no clue what set him off either time. He’s a wonderful little guy, easy to handle, show, vet, never shows aggression towards me or anyone else. I have no idea what the issue was.
Whatever the cause of his aggression, you are exposing yourself to great liability if that horse manages to injure another horse. He’s tried twice, you’ve been warned, please don’t risk it again.
ETA You really don’t want to see an aggressive horse severely injure or kill another. It can be very traumatizing to anyone who witnesses it.
This has happened to me.
I wish I had an answer for you. I don’t.
But I have a 20 year old homebred who I have raised her entire life. Up until 13 years of age, I could trust her to be turned out with anyone. She was a social butterfly, got along with everyone.
Then one day, she tried to kill her pasturemate-- a placid 30-something gelding. She would have succeeded if I wasn’t nearby. I thought it was a fluke; she had only lived with him a few weeks, maybe there was a reason…
… then a week later she tried to do it to her best friend who she had lived with for years.
My vets and I went down all the rabbit holes: ulcers, hormonal changes, pain, etc. She spent time at the university hospital. I even had a benign mass removed and biopsied because one vet thought maybe it had metastasized to her brain.
I tried to keep her on solo turnout, but that did not work at all. So she still gets friends, but is absolutely not allowed in any tight quarters with them; if she goes berserk, I want her friends to be able to get away easily. I also won’t turn her out with anyone but my own horses.
While no incidents were as bad as those first two back in 2018, she has done some unexplained damage to herdmates a few times since.
Knocking on wood furiously, we haven’t had an incident in quite a while… years. Still don’t totally trust it won’t happen again, though.
And if you came to my farm, you would never believe this. You’d see a happy herd of horses with nary a mark on them, all grooming each other and sharing hay piles. This horse isn’t even the first one at the gate at feeding time. That’s what makes this all so bizarre.
Cushings and EPM can cause weird behavioral problems without the normal/common symptoms. Also Lyme and Anaplasmosis! I’d run some diagnostics. Just to be sure.
It’s natural for an older male horse to live in a bachelor herd sometimes in his life and drive other males away under certain circumstances (they mature, breeding season comes, he gets a mare etc). If they can’t get away then yes this kind of injury can happen. It’s hard to say what is driving a gelding to think it’s time to do this but that’s almost certainly what is going on in his mind.
It used to be normal to turn all their stallions out together in the winter and there is never a problem as long as they are separated early in the spring.
Be that as it may, the behavior of feral stallions doesn’t explain the sudden aggression of mares or geldings against other horses. It really is a puzzle and there are probably many different reasons why this occurs.
We, as caretakers, have to realize that some horses become aggressive enough to be a danger to other horses and can’t be managed as we wish they could be.
I’ve had it happen twice…once w a gelding who tried to kill his friend. The other was a mare-same thing. The owner of the gelding understood her horse had to be in a field alone. The mare’s owner did not understand that so she left.
I’ve never had a boarding barn but I can’t imagine anyone in that business taking such a risk with other people’s horses.
You are a good caretaker. 
Yes, I know of a similar situation. The horses belonged to one family and were older geldings that were the kids’ retired show horses. One of them was definitely the Head Honcho but from what I understood, there was never any bad behavior beyond occasional horse herd bickering. Until one evening Head Honcho hauled off and kicked the crap out of one of the other geldings, fracturing its leg and requiring euthanasia. Head Honcho guy is now living out his final days in his own private paddock close to, but securely separated from, the other geldings.
Some horses can be incredibly mild mannered when working with a human, yet aggressive and territorial when navigating herd dynamics. While that may be the case with this offending horse, it’s still a good idea to pull some bloodwork. You never know what might show up, including Cushing’s.
And as @skydy suggested, I would not put him in with horses owned by other boarders. If I had a horse turned out and that horse was grouped with mine, and became aggressive again and injured my horse, I would be furious. So please protect the other horses and yourself!
I would keep him on solo turnout after this myself, he can have neighbors but not pasture mates. He’s decided this is something he needs to do twice now, he will most likely do it again. And he’s big.
I had a riding instructor who had a horse that would randomly savage other horses. She had no clue what caused the issues, but she kept him in a private paddock. However, he was fine on a lead rope and could be handled or ridden around other horses.
Perfectly healthy, gorgeous beast of a horse he was.
I leased a horse with a similar issue. She pinned my horse in a corner and beat the daylights out of her with her back feet. It looked like someone had taken a bull whip to my horse. Just covered in cuts and bruises. She must have been kicked 30 times.
I told the owner and she said “oh she’s done that before. She knocked the other horse down and was standing on top of it and if we hadn’t have been there she would have killed it.”
She was a great riding horse but I never put her in a shared paddock again.
I might have done to the owner what her horse did to yours after that comment. I’m so sorry!
I wonder if there have been any real studies about sudden aggressive behavior in horses.
Could one of our resident Veterinarians let us know? All I can find online is speculation IE it could be pain, territory, hormones etc… @Ghazzu?
My late DH, who was a veterinarian, speculated that the aggressor senses a weakness in the horse they attack and attempts to cull them out of the herd. I’ve no clue. In the situations I’ve experienced, it was a stronger, younger horse attacking an older one.
I expect there are multiple causes.
Fear, territorial concerns, pain, hormones.
My guess is that most likely if these horses were out on a range the target horse would merely be driven off, but we have fences.
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It can be a pretty terrifying thing to see, though.
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Years ago, my old stallion, who was incredibly laid back, and who was turned out in the company of my old gelding, finally got tired of the gelding’s play harrassment and went for him.
His ears disappeared into his neck they were so pinned.
Mouth wide, head and neck extended–he was out for blood.
The gelding realized he’d gone too far, and ran.
They were both grazing on the backyard grass at the time, so when it finally occurred to the gelding to run into the barn, I managed to slam the stall door before the stallion got by me.
The stallion immediately reverted to his quiet self, but the gelding stayed bug-eyed for awhile.
I never saw a repeat of the incident–I think the stallion had made his point.
(“Listen, you little SOB, I’m the one with the testicles!”)
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there’s evidence that aggression in dogs can be related to low serotonin levels, and it has been speculated that this could be a factor in horses in some cases.
I have had two situations where my horses tried to kill another horse in turn out. It was the same horse they went after, by different horses. She was very kind, very submissive and totally healthy and sound.
I was thinking about this thread last night and realized my aggressive mini had been gelded late, as was the two he went after. “Killer mini” was gelded when he was about 7 and may have been used for breeding. The other two were 10ish from what I’ve been told.
Then I remembered about 25 years ago I had another pony that went after my warmblood in the same manner, he was out for blood and it was random. He drove the big guy into a stall and clobbered him, we had all we could do to get him off. He was a maniac and off the rails. Another case of a really pleasant, easy going pony gone wild. I hope it’s not my management 
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