Coulrophobia- Fear of clowns
For the vast majority of people the thought of seeing a person painted in a white face, big red lips, a funny wig and over-sized shoes acting silly is a very pleasant reminder of childhood and makes on laugh or feel good. Clowns are are a common feature of circuses and children’s birthday parties. However, there are some people with morbid, intense fear of clowns called coulrophobia. Often times this fear would have been brought about by a bad or traumatic experience with a clown. Perhaps, as a child, one could have had a traumatic experience with a clown at a party where the clown included him or her as part of the act and was laughed at and that person felt embarrassed. Another cause might have been being scared of the face of the clown and has carried on the fear with them even as an adult. With films like Batman, it is not an uncommon thing to associate characters like the Joker as being evil and thus all clowns are evil.
This also fits with the use of clowns by horror movies that showed clowns as the killers or evil psychopaths. The use of clowns in this way is the filmmaker using what would seem a perfectly safe and harmless character as the bad guy as a surprise that the audience would not expect.
This fear can be helped with a combination of hypnosis and psychotherapy with the possible use of age regression to help the younger self become more comfortable about the existence of clowns.