Sudden aggression in horses

[guest@endurance.net]

bb bkcbs@hotmail.com
Has anyone ever had a perfectly social horse turn nastily aggressive
toward a pasturemate? My 12 year old gelding loves interacting with
horses, has always had a soft spot for mares, and loves to engage other
horses in play.  He has usually been near the top, or at the top of any
pasture hierarchy, but has never actively bitten or kicked or gone after
other horses..until today.  He terrorized a mare he  has regularly been
turned out with for the past two years.  Human intervention was necessary
as he was chasing her, ears pinned flat back, biting and kicking her, and
she was terrified. This is wasy abnormal behavior, such that I'm going to
call my vet and see whether he might have rabies, or some blood
abnormality.  Any ideas, anyone?