A horse that kicks can be dangerous. Even if your horse is merely kicking at a biting fly, and you happen to get in the way, you can be hurt. Some horses develop a bad habit of kicking and can be a menace on the ground, or while being ridden or driven.
Why Horses Kick
Horses kick for a number of reasons. As mentioned above, a horse may kick at biting flies around its legs and belly. A horse will kick at its belly if it has colic. They may kick or stamp if something like a prickly weed tickles their legs or belly. Usually, these aren't really powerful kicks—after all, if the intent was to get rid of the discomfort, they could further hurt themselves.
Horses are often seen kicking at each other in the pasture. When playing, these won't be powerful kicks, and they'll rarely connect with another horse. This can simply be a display of high spirits, often seen as the horse gallops and bucks to burn off energy.
Horses also kick to defend themselves, and these kicks are often powerful and well aimed. Horses may defend themselves by kicking when they feel another horse is getting too close to its food, its foal, a special herd mate, or if another horse is acting aggressively towards it. In the wild, horses use powerful kicks, often with both back legs at the same time, to ward off predators. A mare may kick at a stallion if it is not receptive to being bred.
This defensive instinct may explain why some horses kick when they become alarmed—such as when a person, dog, or another animal 'pops into view' behind the horse. Or if a piece of equipment comes loose and drags behind or alongside the horse, it may react by kicking at it. A horse being trained to pull, may kick at the equipage, unless it is introduced to it slowly, and allowed to get used to the sight and noise of a horse-drawn vehicle.
When Kicking Becomes a Problem
Kicking while being handled, ridden, or driven can become a dangerous habit or vice. Somewhere along the line, the horse has learned that kicking is the best strategy to rid itself of something it dislikes. It then becomes a habit that the rider, handler, or driver must always keep in mind. Some horses get antsy when another horse is ridden too close behind and may kick to warn the other horse away. This causes a problem when the horse is being ridden in a group, or in a crowded area like a horse show. Both the horse and the rider (and spectators) are then in danger of being injured. I do know of incidents in which a horse has kicked at another, and the rider has taken the brunt of the blow.
Some horses dislike dogs, cats, or ponies and will offer to kick them anytime they get too close. Horses that have been hurt while being saddled up or having the girth/cinch done up quickly, will often 'cow-kick' in anticipation of being pinched. My daughter's nose was broken in one such incident. As she bent to reach under the horse to do the girth up, the young horse reacted by cow kicking, catching her in the face.
Some horses will kick out in defiance. This is a show of disrespect. These kicks are aimed towards you, but the horse knows it isn't within range to connect. This often happens while lunging or working in a round pen. Some horses kick the walls of their stalls when they are bored or impatient.
How to Deal with a Problem Kicker
If your horse seems to be a habitual kicker, there are a few things you can do. If you're out, tie a red ribbon on its tail to warn other people that the horse is known to kick. If you're riding in a group, ride at the back of the pack, and make sure others know of its habit. Teach your horse to respond to leg aids. A horse that is moving forward is less likely to kick. In a situation where another horse comes too close behind, you will be able to swing your horse's hindquarters to one side or another so your horse, even if it does kick out, can't hit its target.
Handling a horse that kicks takes extra caution. Any time your horse is in public, it should be wearing a red ribbon. Anyone who must work around the horse should be aware of the horse's habit. You and anyone else that must come near your horse must stay out of range of those hind legs. If you're in a public place like a horse show or fair, you need to position your horse away from foot traffic and other horses.
Learn to Watch Body Language
Most horses will warn you with their body language before lashing out. So, in addition to knowing what situations may trigger a kick, you need to understand its ear, head, and body posturing that may occur before a kick. Whether the horse is aiming its blow carefully or being defiant, you need to recognize the signs of an impending kick and give the horse something else to think about.
You may be able to lessen the vice by desensitizing the horse. If it seems to be scared and kicks out at a specific thing, you'll need to work gradually to get the horse accustomed to that thing. If it cow-kicks while being saddled, you need to be consistently gentle and slow about the process. If the horse habitually kicks at others in the pasture, it may need to be separated if it's causing the others injury.
One way to deal with a kicker is 'kicking chains.' A short length of the chain is strapped to each hind pastern. This is based on the theory that if the horse kicks, it will feel the chain against its own legs, and likely be dissuaded from kicking out. A few things can go wrong with kicking chains. If it scares the horse, it may exacerbate the problem, and subsequently applying any boot or wrap may become an ordeal. Certainly, the first time they're worn, the horse may react violently. Some horses get used to them, and they may even become ineffective. Or, the horse won't kick when the chains are on, but kicking may remain a problem when they're off. The chains could get hooked on a horse's shoe or a wire fence (unlikely but possible). Don't use kicking chains while riding. If you decide to try kicking chains, proceed with extreme caution.