Aggression is a natural behavior horses will exhibit when wanting space, protecting themselves, and protecting others. However it should never cross the line into aggression that is dangerous to both humans and other horses! If aggression does cross that line, then we need to find the root cause/s of why.
Where to Start
If you are dealing with a horse that is dangerously aggressive, I know it is scary and a sad place to be. But there are tons of areas to start looking into to figure out where the aggression is stemming from and how to resolve it! And please do reach out to me or another professional to help you fix this issue safely! Let's get going!
Right off the bat, if you have not already get your vet out to do a thorough exam of your horse. Many physical issues and diseases can cause a horse to become aggressive and will need medical intervention in order for it to be resolved.
Sometimes, it can even be hormonal issues especially if you are dealing with a stallion. But even mares can have hormonal imbalances! Mares can end up producing testosterone if they get a tumor on their ovary or on their adrenal gland. I actually knew a mare that was always aggressive and it was not until the vet finally did an ultrasound of her ovary that they found a massive tumor on it!
If your vet finds that your horse is healthy and that there is nothing medically showing up, then it is time for you to grab a shovel and dig deeper to find the root cause!
Horse's all have instinctual needs that need to be met in order for them to be content. Those include access to forage throughout the day, open space to roam, and friends they can interact with.
If a horse does not have enough forage they can become aggressive especially when you feed at your allotted times during the day. Horses should not be without food for more than two hours because it slows their digestive tract! This can lead to colic, stress, ulcers and changes in mood.
If a horse does not have enough room to roam and play they can become distressed, aggressive, and it can lead to all sorts of health issues. While on the flip side, if you put them out with other horses and the space is not big enough to hold them all, aggression will most certainly show!
Horses are social animals and are meant to live in herds. If a horse does not have any friends they can interact with, a host of behavioral issues can show up! For example, with stallions many people want to separate them from all horses for fear of aggression and obviously unintended breeding. But what has been found and shown is that some stallions will self-mutilate due to not having social interactions with other horses! When stallions are put out with geldings or other stallions this issue will not occur and their behaviors will become balanced!
Make sure that all of your horse's needs are met! If some are missing, then that could be the root cause of aggression!
Previous Experiences (Some Trauma May Be Present!)
Sometimes aggression ends up forming from the horse's personal experiences. For example, if a horse has had a painful injury and then a painful treatment, they can become aggressive and protective to that area on their body. They can also become aggressive to those who helped treat the injury. If this occurred, then you have to slowly get the horse to trust people around that area of their body.
Sometimes a painful experience can become stuck in their memory, especially for abused horses! A bad trailering experience can easily make a horse aggressive whenever they are asked to go near or in a trailer. Or a horse can be aggressive to only men if they were previously abused by a man. Again rebuilding trust along with patience is needed to help release trauma like this.
Aggression Learned Through Observation/Association
Horses are amazing learners and many times we forget they are always watching and figuring us out! Well this can sadly lead to us enforcing aggressive behaviors by accident! A perfect example is during feeding! If a horse starts to pin its ears or bit the air and you rush to feed it to make it stop, you have reinforced this aggressive behavior. If you think your horse's aggression may have been learned from you by accident, you will have to take the time to help them unlearn it!
Sometimes horses are aggressive due to how they were raised. A foal should be socialized very early to people and other horses. They should be handled as soon as possible but given the time they deserve to be a baby! However sometimes things do not go as planned, like a mare rejecting their baby. When this happens, a foal loses the most precious gifts their mother can give them, experience and discipline. A person can only do so much in disciplining them and teaching them about life. It is so easy for a foal to grow and become aggressive and dominant without the guidance of their mom or another horse.
When working with an aggressive horse do not get aggressive back since horses are mirrors and they will throw more anger right on back, which can make the situation even more dangerous than it already is. If you are nervous about helping your horse you need to reach out to a professional, like a trainer or horse behaviorist like me, to help you work through it safely.
Make sure their needs of forage, space and friends are met. With the friends, if your horse gets aggressive with others, start with having them in a paddock next to others so that they can sniff and touch noses if they feel. Slowly work them up to being out with one friend. Aim for a buddy who will not let them get away with their behavior. This also goes for any foals that were not raised by their moms and even those that were with their mom! Get them out into herds so that they get more discipline!
Personal Experience: One of the barns I worked at had a really sweet older horse that would just terrorize other horses he was out with, until we put him in with two very alpha-like horses. They put him in his place and we were even able to put out less dominant horses into their herd and he would not terrorize them since the alphas would not allow it!
For food, always make sure in herds that there are enough forage piles separated by a good amount of distance so everyone can eat and no one gets food aggressive!
If aggression is being shown towards humans, always work with the horse from behind a fence/stall door until they get more trusting and you feel confident!
Watch how close you have to be until aggression starts to show and then back away when you see the slightest change to a positive attitude. Repeat this until you are able to get right up to the horse and touch them with no signs of aggression.
No matter what, there is a root cause to a horse's aggression and it just may take some time to figure it out. There are many other things I could tell you to do or try. But above all, you need to stay safe, do not be aggressive back at them and please find someone to help you on this journey!