What phobia is this and why do some children suffer from it?

My friend's daughter is 12. Since she was a toddler, she's always been deathly afraid of people in costumes. The fear increases as the costume gets more elaborate. (we're talking sports/ business mascots, Disney characters, and such...nothing scary) She can sort of tolerate it if she can see the persons face, but put on a mask or a head-piece, she goes ballistic. She's ok when the costume is empty. They've tried to have someone they know put on a costume in front of her many times, but as soon as the face is covered, the girl experiences sheer horror, as if the person in the costume no longer exists. While I feel sorry for the girl, I am fascinated by her reaction and would like to know more about it. I also understand that it is not uncommon for children to be frightened, but it is rare for a 12 y/o girl to still foster this fear. Any ideas?