Summarize:

If you’re riding a horse and you start to feel it getting excited, or something on the trail spooks it, your instinct might be to tighten your grip on the reins. However, this will make the horse feel even more anxious and antsy. Instead, try to loosen your grip on the reins, which may make the horse a little calmer.  Also, try not to clamp your legs against the horse’s side. If you overreact by yanking on the reins and yelling, you’ll reinforce that the horse had something to be afraid of in the first place. Since your horse will pick up on your body language, stay as calm and relaxed as possible. You don't want to accidentally cue your horse to become more nervous. Give the horse a command to slow or stop, like, “Whoa,” "Easy," or “Calm.”  Use the same command you used when training the horse, and try not to shout, even if you’re feeling nervous.  It’s important to be consistent in using verbal commands during training. That way, when you’re in a situation away from home, the horse will know exactly what you expect it to do. Most of your instructions to the horse should be through your body language and your use of the reins, but verbal commands can be useful as well. Don’t yank or snatch at the reins, but pull one side firmly and steadily until the horse starts to turn. You can turn the horse to either side, but if it has one side that it favors during training, you may want to start by turning the horse that way.  Turning the horse’s head will stop it from moving forward, and it will also cause it to lower its head, which is a cue to the horse to calm down. Leave plenty of slack in the other rein, but don’t drop it. As you’re pulling the rein, press in with your leg on the same side. Squeeze your leg into the horse's side firmly, but don't kick sharply or you could startle the horse, making it harder to control. This is called the hips over technique, and it should cause the horse to step sideways to try to ease the pressure you're applying. If the horse doesn't respond within a few seconds, squeeze your leg against its side harder until it moves. Stepping sideways will discourage the horse from moving forward, and it may also distract the horse from whatever has excited it. Don’t hold pressure on the rein too long, or else the horse will start to fight against it. Instead, once you feel the horse take a big step to the side, ease up on the rein and hold it loosely in your hand. Although the reins should have plenty of slack in them, you should still be holding them securely. While a single turn might be all it takes to calm a well-trained horse, a more inexperienced horse may still be feeling excited. If it’s still jigging, pull back on the opposite rein and apply pressure with your leg on that side. You may need to repeat this a few times, alternating which side you turn toward each time. After you’ve calmed your excited horse, keep it distracted on the ride by giving it plenty to do. Frequently adjust your riding speed, and have the horse ride around objects along the trail so it will have to constantly mind what it’s doing. This may help shift its attention away from whatever excited it in the first place. You can also move ahead or behind other horses if you’re riding in a group. An over-excited horse may not immediately respond to a calm-down cue and the hips over technique. If the horse is panicked, or if it rears, bolts, or otherwise causes you to feel that you’re in danger, you should exit the horse quickly and move away. Remove your feet from the stirrups, push yourself up off of the saddle with your hands, and swing one leg over the saddle. Then, push away from the horse slightly as you slide down to the ground. Even if you’re falling, you can avoid injuries by tucking into a ball and landing on your shoulder, then rolling away from the horse. Do not put your arms out to catch you while you’re falling.
Ease up on both reins for a moment. Give the horse clear, calm commands. Pull one side of the reins to pull the horse’s head around. Use your leg to push the horse's hindquarters to the side. Release the pressure on the rein when the horse steps sideways. Repeat on the other side if the horse hasn’t settled down. Give the horse constant instructions to keep its attention occupied. Dismount if you’re still having trouble getting the horse under control.