How to Stop a Horse Rearing
To stop a horse rearing when you're riding it, move your hands and body forward and relax your grip on the reins until the horse has all 4 hooves on the ground. Next, reapply gentle pressure to the reins and squeeze your legs together to get the horse moving forward. Then, once the horse is moving forward, gently pull on 1 side of the reins to make any additional rears less dangerous.
Check to see if the horse is rearing out of pain. A horse may rear in response to pressure on the reins if its bit is too harsh or is causing it pain. Make sure its bit is the proper size and fit in the horse's mouth to eliminate this potential cause of the rearing. You'll want to make sure that the rings or metal connections at the corners of the horse's mouth don't pinch its lip and that the center of the bit doesn't hit the roof of the horse's mouth. If you use a bare metal bit, consider switching to a rubber or plastic-coated mouthpiece, as these are more comfortable for horses. If a painful bit is what's causing your horse to rear, you'll probably notice the rearing behavior happening most often in response to you pulling on the reins. Your horse may also rear due to pain caused by an ill-fitting saddle. Have a professional check the fit of your saddle to determine if there is any pinching or unevenness in the areas where the saddle contacts your horse. Your vet can also help you check for saddle-related pain. Determine whether your riding style is causing your horse to rear. If your riding style makes your horse uncomfortable or causes it to become confused, this may cause it to rear out of frustration. Analyze your style to make sure you're following proper form and not doing anything to upset your horse. For example, if you kick the horse forward while simultaneously pulling back on the reins, the horse may be confused as to what you want it to do. Make sure you're properly balanced in the saddle, as well. If possible, ask a riding coach or someone familiar with riding horses to watch and critique your riding style. They will be able to let you know if there's anything you need to do to fix the way you ride your horse. Avoid putting your horse in situations where it feels afraid. Horses sometimes rear out in the field when they get scared or uncomfortable around something on the ground. If you come across something your horse doesn't like, don't force your horse to confront it or make it feel like it has no way out of the situation. For example, instead of forcing your horse forward when it doesn't want to move forward because of something on the ground, see if there's a path around the frightening object or animal that you can take. Horses are spooked by a variety of stimuli. For example, some horses may be spooked by small critters, while others might be scared of bubbling brooks if they've never crossed water before. Pay attention to what spooks your horse. Once you identify some triggers, you can make time later to work on desensitization training (in which you gradually expose the horse to whatever it fears in a safe and controlled environment). Train your horse to maintain respect for you. In addition to these more immediate stimuli, another reason that a horse will rear is partially out of disrespect for its rider. Work with your horse every day so that it knows it can trust you, while also refraining from punishing your horse for “being disobedient.” Instead, simply reinforce the horse when it behaves the way you want it to. Your horse will better respect you for telling it what TO do rather than only chastising when it doesn't do what you want it to. Ask a professional for help if you can't fix the rearing yourself. Rearing is very serious, so if you're not able to eliminate the causes of the behavior on your own, you'll need to reach out to a horse trainer who can. Look online for certified horse trainers in your area and ask them about training your horse to prevent rearing. Be willing to ask any potential trainer about their prior experience. Getting a horse to stop rearing is a relatively tricky bit of training, so it's important that whomever you hire to help you has some experience with it. A trainer will work directly with you and your horse to figure out what's causing it to rear and how best to prevent rearing in the future. Stay as close to the shoulder as possible and hold the lead loose. This is the safest position to be in when working with a horse, since it's harder for it to swing and hit you from this position. Loosen your grip on the reins so that the horse doesn't feel like you're pressuring it to move a certain way. This will help the horse feel less like it needs to rear. Standing next to the shoulder also prevents the horse from being able to hurt you with either its front or its back feet. Try not to make any sudden or startling movements. As you move to stay at the horse's shoulder, keep your movements smooth and controlled. Making any sudden, wild, or jerking movements will only startle the horse and frighten it further. Don't make any sudden movements to try to calm the horse or hold it still. Tip its head toward you to control its forward motion. Once all 4 of the horse's feet are on the ground, tip its nose down and toward you to keep its attention on you. Lowering your horse's head this way will not only help it to relax and keep it from rearing up again, but will also prevent it from moving forward before you want it to. Horses tend to lift their heads and tense their neck muscles when they're anxious, so lowering your horse's head will thus cause it to loosen its neck muscles and relax. Don't try pulling your horse's head down as a means of physically preventing it from rearing. If the horse decides to put its full force into the rear, it will almost certainly overpower you. Push on the horse's side with your fingertips to move its rear legs. Since rearing involves the horse lifting its front feet into the air, a good way to prevent this in the moment is to get the horse to move its back legs while keeping its front legs on the ground. Gently push on 1 side of the horse with your fingertips until it moves, then ease up on the pressure. Continue doing this so that the horse moves its rear legs in a circular motion. Make sure that the horse's front feet remain in the same place the whole time you do this. Whenever you cease applying pressure with your fingertips, gently rub the area to “erase” the feeling of the pressure from it. Your horse will appreciate this kind gesture. Steer the horse in a forward motion using light pressure on the reins. Once you feel like your horse is firmly on the ground and isn't about to rear again, use clucking sounds and gentle pressure on the reins to get it moving forward. The movement will not only prevent your horse from rearing, but will also allow your horse to feel relatively free and in control over its motions. It's important that you use gentle pressure and clucking rather than a hard forward tug on the reins, since you want to avoid making your horse feel frustrated or uncomfortable about what you want it to do. Stay calm and avoid pulling back on the reins. While yanking on the reins is an understandable response to being on a rearing horse, this will actually only exacerbate the situation and may even cause the horse to fall backwards. Instead, relax and avoid doing anything that may add to the horse's stress. Don't lean your body back or to the sides, either; this may also cause you to fall off. Move your hands and body forward, loosening the reins. While the horse's head is in the air, tilt forward to avoid falling backward and off of the horse. At the same time, relax your grip on the reins, both to avoid pulling back on them and also to eliminate 1 potential cause of the rear itself. Pushing your hands forward and toward the horse's mouth is a good way to loosen your grip on the reins while also tilting your body forward. Don't let go of the reins entirely; you'll want to make sure you're eventually able to regain control over the horse. Reapply pressure on the reins once the horse is back on the ground. Make sure you only use gentle pressure, especially if you aren't sure if the bit is causing the horse pain or not. Once all 4 feet are on the ground, squeeze your legs together to get the horse moving forward. Once the horse is moving forward, pull gently on 1 side of the reins to get the horse moving laterally, as well. This will make any additional rears much less powerful or dangerous to you. Slide off of the horse if you feel like you're in danger. Being on top of a rearing horse is very dangerous, so if you're inexperienced or feel like you may get hurt, the best thing to do is remove yourself from the situation. Lean forward to wrap your arms around the horse's neck and slide yourself off of the horse. Make sure you avoid going in front of the horse as you exit it, as its front feet will be very dangerous mid-rear. Move off to the side and get away from the horse once your feet touch the ground. You may need to stop holding the reins in order to successfully slide off of the horse. 
Check to see if the horse is rearing out of pain. Determine whether your riding style is causing your horse to rear. Avoid putting your horse in situations where it feels afraid. Train your horse to maintain respect for you. Ask a professional for help if you can't fix the rearing yourself. Stay as close to the shoulder as possible and hold the lead loose. Try not to make any sudden or startling movements. Tip its head toward you to control its forward motion. Push on the horse's side with your fingertips to move its rear legs. Steer the horse in a forward motion using light pressure on the reins. Stay calm and avoid pulling back on the reins. Move your hands and body forward, loosening the reins. Reapply pressure on the reins once the horse is back on the ground. Slide off of the horse if you feel like you're in danger. 