I admit my horse Porter is hyper-sensitive, but
there may be other horses out there like him.  I
once put an apple-vinegar wash on him after a ride 
around the ranch where I boarded.  It was 2 cups of 
vinegar in 2 gallons of water.  I put it on him as
a fly repellent.  He ended up running around out of 
control bucking then throwing himself on the ground
and rolling. (it was about a 5 minute delayed reaction.)
It appeared to be burning his skin!  I finally caught
him, and was able to hose him off and shampoo him. 
I felt soo bad!!  I can't put any kind of
liniment on him either.  
Anyways, I would suggest trying it out on a small part of 
the horse first before completely treating with vinegar.
Vinegar is pretty harsh - I also use it to scrub the mineral
deposits that build up from the water on my faucets at home.
Stacy 
Morgan Hill, CA