Get to the root cause of aggression and pursue appropriate training to help keep horses and humans safe

The 7-year-old Warmblood was a show jumping superstar—or at least, he used to be, back when riders could get on him. The 17-hand horse became so dangerous his owners were losing hope. That’s when they sent him to the Australian equitation-science-based horse trainer Andy Booth, now based in Chamadelle, France.

“You couldn’t even get him out of the stall without getting kicked,” Booth recalls. The stallion threatened anyone who approached the door. If you managed to get a lead on him, he’d rear to vertical, striking out incessantly toward the handler. In the rare event handlers could get him saddled, he’d buck and twist to throw the rider high and far.

We’ve asked experts to find out why horses display aggression and how to manage them—whether the label is accurate or not—for the safety and welfare of both species.

Normal Aggression in Horses

Horses showing aggression toward other horses—but not other species—are likely just displaying their normal species-specific behavior, says Christine Aurich, DVM, PhD, head of the Graf Lehndorff Institute for Equine Science, in Neustadt, Germany.

Still, domestic horses usually limit aggression to nonviolent threats, says Alice Ruet, PhD, welfare science engineer at the French Institute of the Horse and Equitation (IFCE), in Saumur, France. If you provide enough space—whether in the field or under saddle—horses generally work out aggressive encounters via body language.

“Being highly aggressive is probably not biologically adaptive, as horses would waste energy fighting when they could be resolving issues through subtle behaviors instead,” explains Janne Winther Christensen, PhD, associate professor in the Department of Animal and Veterinary Sciences at Aarhus University, in Tjele, Denmark.

Horse Aggression Toward People

Today’s domestic horses reflect five millenia of selective breeding to avoid aggression. In fact, tamability was likely the first trait people targeted when domesticating horses around 3000 B.C., says Ludovic Orlando, PhD, director at the Center for Anthropobiology and Genomics of Toulouse, in France.

His team recently noted a rapid generational drop in the presence of ZFPM1—a gene associated with aggression—in ancient bones at horses’ earliest domestication sites, in Kazakhstan. That drop occurred well before breeding for height, muscle mass, speed, or rideability. “One way to start the animal domestication process is to have the behavior under control,” Orlando says.

Still, many other genes could be linked to aggression, says Ruet. “Certain personalities might predispose horses to becoming aggressive when poor welfare triggers it,” she says.

Some such genes might linger because they coincide with desirable traits, Aurich explains. “A racehorse probably needs some aggressive character components to become a front-runner, and a cutting horse needs some aggressive behavior versus cattle,” she says. Breeders generally know which lines produce more aggressive horses, she adds.

A genetic propensity for aggression might even lie dormant “within the telos of all horses,” says Andrew McLean, BSc, PhD, Dipl. Ed, owner and director of the Australian Equine Behaviour Centre, in Moorooduc, Victoria. This tendency could be exacerbated by other unknown genetic factors and sex hormones such as testosterone, he says.

Evidence-based management and training styles reduce the risk of revealing latent aggression in horses, McLean says.

What Brings Out Horse Aggression?

Aurich and Ruet agree that interactions between horses appear to have an important role in stirring up aggression. Orphaned, bottle-fed foals and stallions raised away from a bachelor herd face a higher risk of developing aggression. “Being raised in a group apparently helps train their social skills—
including towards humans,” Aurich says.

Ruet says pain is another critical factor that, as in humans, can lead to aggression, even if the horse is simply anticipating it.

Improper management can also encourage aggression. In fact, overlooking horses’ inherent needs can “result in disaster,” Aurich says. This is more likely to manifest in horses that tend to have a stronger fight-or-flight response, she adds.

Managers should aim to reduce horses’ stress and frustration, providing access to friends, forage, and freedom—the classic “3 Fs” of horsemanship, Ruet says. In fact, she found that show horses kept in stalls without access to the 3 Fs became aggressive—or progressively more aggressive—toward people over two years.

Boredom might also encourage aggression, especially in stallions, Aurich adds.

Trainers, meanwhile, should adhere to scientific guidelines based on horses’ abilities to learn, says McLean. Otherwise, methods can cause confusion and frustration.

Is Horse Aggression Innate or Learned?

There’s a difference between truly aggressive horses and horses that have picked up aggressive behavior due to the benefits it provides. Both situations are dangerous and can be difficult to fix, but it’s crucial to know the difference.

Truly aggressive horses act out of negative emotions—roughly equivalent to human anger, says Ruet. In her research she’s even shown that aggressive horses strike their feet harder against the ground under saddle than nonaggressive horses do—like angry humans stomping around. These horses clamp their jaws and flare their nostrils, often remaining highly agitated even after an aggressive act, she explains.

Horses that have learned aggression, however, might appear angry without the accompanying negative emotions. They could have figured out, for example, that threatening to bite makes people move away, says Booth. Or they know leg cues disappear when they buck or rear. These horses often seem calm after acting aggressive.

Let’s go back to that show jumper as an example. After watching him rear and strike out, Booth says he realized the horse wasn’t going for the handler’s head; instead, he was trying to catch the lead between his forelimbs. The handler would always release the pressure, and the horse would walk calmly away to graze. “He’s figured out exactly how to get that pressure release,” he says.

Angelo Telatin, PhD, associate professor of equine studies at Delaware Valley University, in Doylestown, Pennsylvania, says aggression often develops as a learned defense from to improper training. “They’re aware that they can move you,” he says. “If the horse is truly aggressive—with the power it has—you’d be dead.”

Many aggressive horses have not been trained with a method called shaping—learning in small, incremental steps. “If you don’t shape before applying leg pressure for the first time, some horses get stiff in the body; some turn around and bite the leg; some act like it’s flies, and they kick,” Telatin says. Applying stronger pressure usually just leads to more aggression. “Some horses dump you or buck you off, or rear and back up, or bite your leg hard.”

A similar problem occurs with whip use on the ground, Telatin adds. “As soon as horses come out with the fight or flight behavior, they see you back off with the whip or longe line,” he says. “So you’re actually shaping this attack behavior.”

People often confuse learned equine aggression with disrespect—which is beyond a horse’s cognitive abilities—or dominance, Telatin says.

Booth agrees: “He’s not trying to dominate you. He’s trying to rip your arms off so you stop annoying him.”

Regardless of whether true or learned, all aggression probably has its roots in a welfare issue, insists Ruet.

First Up: Relief and Safety

Our sources emphasize that aggression demands immediate attention and effective management.

Instead of labeling a horse as mean, consider his history. “Horses express poor welfare in a variety of ways, and aggression is one of them,” says Ruet.

With stallions, handlers can use enrichment (such as feed balls or toys) to help pass the time—as well as training sessions to stimulate their minds, Aurich says. Critically, stallions need a lot of social contact beyond mere breeding. Handlers should try to provide contact over safe barriers or even a trans-species companion such as a goat.

Aurich recommends veterinarians find and treat any back, foot, or leg pain, gastric ulcers, dental issues, or other sources of pain. “It can be very difficult to identify the cause, but a workup is super important,” she says. “This may take time and a lot of patience.”

Drugs can serve as a last effort to manage aggressive horses, Aurich says. Certain calming agents and pheromones might temporarily reduce aggression, but no current research confirms their effectiveness for this purpose. Analgesics or anti-inflammatories could make some horses less aggressive if the drugs relieve their pain. But even if drugs help, only consider them temporary solutions while working to confirm and resolve the underlying problems, she says.

Meanwhile, people need to focus on minimizing their own safety risks, our sources say. Fence aggressive horses away from high-traffic areas. Use muzzles if necessary.Wear protective equipment such as helmets, vests, hard-toed boots, and even shin guards when working with these horses.

Next: Shift to Positive Associations

After resolving any health and welfare issues, restart the horse-human relationship with patience and positive associations.

“Just get out there and spend some time with them without working them,” Ruet says. This doesn’t mean horses don’t like to work—and, in fact, work can be a positive experience. “But if the only time horses see humans is because the humans want them to work, that’s not ideal,” she says. Aim for quiet, comfortable encounters with food rewards and mentally stimulating experiences.

Lasting Behavior Change

When the horse’s perception of humans improves, skilled trainers can work in teams to gently retrain the horse.

For Booth, trainers must always “make the right response comfortable and the wrong response uncomfortable,” he says. He quickly retrained the jumper that always presented his hindquarters to the stall door by waving a small flag in front of the door until the horse turned; when the horse approached the door, Booth gave him a treat through the bars. Within a few sessions, he could safely open the door and halter the horse. He would then lead the stallion out to graze, creating and maintaining a positive association with the stall door opening.

Telatin retrained a mare that would bite anyone coming into her stall by clicker-training her to play with a ball instead. “Then you could halter her or do whatever you wanted,” he explains.

People can use their elbows to block biting horses in swift movements, say Telatin and Booth. Elbow pads can be very helpful here, Booth adds. For bucking horses, you can start retraining with a dummy tied to the saddle—but eventually a skilled rider needs to get on, stay on, and ride out the bucking until the horse offers the right response—calmness. Importantly, riders must release pressure at the calm moment to reinforce the correct response.

Still, retraining aggressive horses can be particularly dangerous—especially because they tend to intensify aggressive responses before trying other behaviors, Booth says.

The Spontaneous Return Factor

Once horses have learned aggression works in their favor, it’s difficult to make that behavior disappear—despite the best science-based retraining methods. “The behavior can always spontaneously reappear, even years later,” Booth says.

Sometimes such horses can be rehomed with someone who understands the risks and knows how to avoid them, he says. In certain cases, though, the most humane decision might be retirement or euthanasia.

Take-Home Message

Aggressive horses are usually made, not born, and many aggressive behaviors have their roots in welfare issues. It can be difficult to de-train aggression because the behavior is so effective in removing pressure. Handlers should use science-based management and training to avoid pain, frustration, and other welfare problems that might lead to aggressive behavior. If such aggression does develop, take it seriously from the beginning by looking for welfare issues and working with a science-savvy professional to retrain the behavior before it sets in for good.