You don't give me much
detail about your horse and his care, so I'm also answering this for other people
that have horses with a similar problem.
horse that reaches out to bite horses or people that pass by their stall is usually
"saying" two things:
1. I am
bored in this confined space.
away from this space, it is mine.
as to #1, horses are inherently nomadic, that means they have evolved over millions
of years to be roamers, constantly on the move in search of food and water and
while doing so, gaining regular, constant low-level exercise. Their diet consisted
of prairie grass. A stalled horse has had his world turned upside down. He now
gets all his feed delivered to him, often consisting of rich feeds such as grain
and alfalfa. And he rarely gets out of the stall. When he is taken out, it is
often to be ridden for a short period of time (one hour) several times a week.
So it is no wonder many stalled horses are jumping out of their skins with boredom
and invent games to amuse themselves and pass the time. So the first time a horse
that lunges out at passersby gets a reaction (shrinking, ducking, dodging), he
has learned that this is a fun interactive game. And he will continue because
it gives him something to do.
As to #2,
since the stall is where a horse is fed, it is his private guarded space and he
could be acting out his pecking order by saying "Stay away from my food and
the area where I eat." Horses aren't generally territorial like dogs are
except when it comes to feed.
bottom line. If a horse is biting out at people and horses, that horse needs more
exercise and interaction and probably less rich feed. The more naturally a horse
is kept, the less apt he is to exhibit stall vices.