problem mare

[Pamnetus@aol.com]

I have a mare that I can let anyone ride.  She is great with kids,and does 
anything you ask with no problem EXCEPT on a ride she pins ears back and is 
positively horrible when another horse passes.    Have tried all I can think 
of to get her out of this dangerous habit.  She is not competitive.  At the 
start  of a  couple of rides where there has been grass on the side of the 
trail she has stopped to graze while horses  are flying by and she could care 
less about them, it's when we are trotting along, so when I hear horses 
coming from behind I pull off the trail.   She is on the bottom of the 
pecking order and I think when a person is riding her she gets brave? I don't 
know.   Any suggestions would be truly appreciated.