My horse is a bit like that - I didn't do this intentionally, its just that she was abused when she was young and then another owner (who was nice to her) didn't do much work with her and left her out in the paddock for a year without doing much work with her. I've heard that people lost their tempers with her and whacked her on the head with a metal bar when she didn't behave
I think that because I've had her for 4-5 years, she has learned that there's one person she can trust. She is afraid to have other people saddle her - we were trying saddles on her at PC and she wouldn't let anyone get near her with a saddle. They ended up giving the saddles to me and she stood perfectly still for me to put them on. It really makes you wonder what people must have done to her.
When someone else rides her, she isn't nasty, but she looks really worried and confused. I guess she's so used to me and still remembers the bad stuff that happened, she gets rather nervous having someone different. I'm starting to let my sister ride her, but only very slowly so my sister doesn't get hurt and my horse doesn't totally get freaked out.