The first time a horse sees the clippers, a water jump, velcro straps, or even a round pen or a trailer, he thinks they are a threat to his life, so getting angry or attempting to restrain him will only make him fight harder for his survival.
Some horses will reach a point of being very aggressive while others will stand and freeze.
There must be a delicate balance that has to be struck when handling phobic horses. This involves being direct enough but at the same time, sympathetic enough to help him dealing with the situation. There are a number of exercises that can help a horse to deal with such a phobia. It also requires a great level of sensitivity.
To deal effectively with phobias takes many years of experience and should not be undertaken lightly.