Rearing is not only frightening for the rider, but it is also extremely dangerous for both parties. So what could cause a horse to exhibit such behaviour?
Consider when the horse rears. Does it happen for no apparent reason during schooling sessions or is it an evasion to one particular exercise or set of aids? I once had a horse which reared as part of a napping behaviour whenever he was asked to leave the yard on his own; the stronger the aid applied to ask him to walk forward the higher he tried to rear. Eventually, he overcame his fear of hacking out alone and the rearing disappeared.
If your horse displays this behaviour, always have his teeth, back and tack checked out by the relevant experts before you do anything else. This is particularly important if your horse has never reared, bucked or napped before. If your horse is young and still growing or has gained or lost condition recently, it could be that he has changed shape so that his saddle is no longer sitting comfortably on his back. It does not take a huge physiological change to radically change the fit of a saddle. Perhaps he has a troublesome tooth which is being aggravated by his bit. Maybe his bridle has been reassembled slightly differently after you've cleaned it and the bit is now either too high or too low in his mouth and consequently causing him discomfort. Your horse may have been cast in his stable or have slipped over whilst clowning around with his chums in the field and could be suffering from a pulled muscle or a knock you can't see.
If the habit persists after all the usual suspects detailed above have been eliminated, the horse must be reacting to something the rider is doing whilst on his back. Look at things from a horse's point of view. When a horse allows a rider on his back he overcomes a massive psychological hurdle: the instinctive fear that equates weight on his back with the attack of a predator. If the rider is tense, unbalanced or clumsy, the horse's instinct kicks in and he succumbs to his natural urge to get rid of the source of his anxiety.
So, check your position in the saddle and the aids you use when your horse gets tense and the rearing problem occurs. Knowledgeable eyes on the ground are vital here but you can also ask yourself the following questions:
Am I inclined to be left behind during transitions from one gait to another? Does this cause me to bump down heavily or too far back in the saddle?
Do I sometimes catch my horse in the mouth or use the reins for balance?
Do I sometimes feel as though I am wobbly or unsteady?
Do I get tense, for example when asking for a canter transition?
If your honest answer to any of the above questions, then it could be this that is causing the horse to become tense and sparks off the rearing behaviour. Try asking a more experienced rider with a secure seat who will not be nervous to ride your horse for a while until he settles down. In the meantime, you could continue working with your horse on the ground; long reining or lunging perhaps to help re-build your mutual trust and bond before you get back in the saddle.
It's possible that the horse is confused by some of the aids you are giving particularly if he is a youngster or new to you. Make sure you're not sending him forward with your leg but forgetting to soften your hand at the same time. If a specific set of aids causes the upset, try softening them and asking more quietly. Some horses are more sensitive than others and will react from a "whisper" rather than a "scream".
If your horse is a mare, her behaviour could be linked to her season cycles. Sometimes the rush of hormones can affect mares' behaviour under saddle. They can become particularly "clingy" to their field companions and this can lead to napping and rearing when they are separated. If you suspect that this may be the case, try putting your mare on Regumate for a couple of weeks to see if it helps. Sometimes this can be all you need to settle things down again.
Horses are very quick to learn and unfortunately if rearing is being used by the horse as a rider frightening evasion to work, you may find it a difficult habit to break. You may have to go right back to basics to break the cycle which leads to the behaviour.
There are plenty of so-called cures for rearing ranging from pulling a horse over backwards when it rears to hitting it between the ears with a schooling whip or even an egg! Such drastic action does not work and will only serve to frighten the poor horse even more. If you find yourself at a loss; refer your horse to a professional behavioural specialist. The expense will be well worth it in the long term for both you and your horse.