Am I out of line to yell at someone else's horse? I feel bad doing it because I assume his aggression stems from abuse but I also don't like having to worry about him sinking his teeth into my animal every time I walk by.
Out of line? Not at all.
There are plenty of lungers and nippers at my barn.
First time, I notify the barn manager that they need to put up a stall guard to prevent this from happening.
If a stall guard isn't put up, I give the offending horse a stern look when walking past. If that doesn't immediately make him retreat into his stall, I tell him to get back (speak, yell... whatever it takes to get him well back into his stall.
If he actually does bite at me or my horse, I wouldn't hesitate to give him a good smack across the mouth. I would do the same to my horse. Biting is very serious, and should never be tolerated.
Keep in mind that horses doing this to passers-by from their stalls is very dangerous. Other horses learn the behavior and pretty soon they all do it. Horses also learn to expect it as they walk by and will start to "jump" out of the way. Not a behavior you want from horses walking down an aisle.
If an owner or barn manager will not put up a stall guard of other restraint, it becomes your responsibility to insure the safety of you and your horse. If this means yelling at the offender, you are well within your rights.