Horses that spook or shy a lot can be very dangerous, not only to themselves but to others as well. Due to the fact that horses are prey animals they have a flight or fight response to perceived threats; this makes them prone to running away from objects they view as threatening. This excessive shying usually results in bolting and you have a problem horse on your hands. The offending object could be something as harmless as a wheelie bin or plastic bag, but to the horse it is a scary object and he wants to get as far away from it as possible!
There are some things you can do to desensitize your horse. This problem horse training is ideally started when the horse is young and in training as it will ensure a spook proof horse. Begin by lunging the horse for a few minutes to allow him to work off any excessive energy. Leave a halter and lead rope on the horse and approach him slowly with an old saddle pad or blanket allowing him to sniff it. Work at the horse's pace and if he spooks or takes a step back, introduce the blanket again slowly until he is comfortable having it close to him.
Once he's more at ease around the blanket, you can start moving it around and swinging it slowly. This will help desensitize the horse against flapping objects. The horse will most probably feel uncomfortable and may even shy, so be patient and work with him until he is accustomed to having the blanket swung around him.
The next step is to touch the horse with the swinging blanket, gently swing it onto his back and all over his body. Once the horse accepts the blanket on his head you're halfway there! Place the blanket by his feet to help desensitize him to objects under his feet, this is an essential part of the desensitization exercise. That's it; reward him profusely for a job well done!
The next desensitizing exercise you should try is to use a soft rope and work with it in much the same way as you did with the saddle pad or blanket. This teaches the horse to tolerate having a rope moving around his legs and body. Desensitizing a horse to this type of sensation can prove invaluable if tack fails and the horse has pieces of tack flapping around him. This makes the horse much safer in dangerous situations when tack breaks. Throw the rope on the ground and in-between the horses' legs and belly until he is no longer afraid of it. If the horse allows you to move the rope around his face and head, you've successfully completed this exercise.
Last but certainly not least is the plastic bag. Many, many horses are afraid of plastic bags and this exercise will help eliminate that fear of plastic bags in your horse. Show the bag to your horse and allow him to smell it, then tie it to a long lunge whip and wave it around while holding your horse on long lead rope. The horse will inevitably shy and try to get away from the terrifying object. Shake the bag around the horse until he becomes comfortable to having it around him, once this happens, touch him with bag all over his body. Move it around his body and under his belly, when he allows you to touch his head with it, you're done.
Work at your horse's pace and always end your training session on a positive note and with a lot of pace. Never pressure your horse in a difficult situation as you may cause him irreparable psychological damage. Do these exercises regularly to ensure the horse remains desensitized.