This 4'3" brilliant exercise rug was used on a 14'2 hh New Forest pony.
Even though these exercise rugs are not deemed waterproof, I found that when going out in the rain the horse stayed amazingly dry and never once did it sweat up under the rug, even on dry days.
Thermatex exercise rugs are truly unbeatable and also long lasting.
They cost £160 new, asking for £90.
Collect from Minstead or post at cost.

From their website:

The Nordic Exercise Rug wraps around the saddle to provide full coverage over back and quarters during exercise.
Crafted from our unique wicking fabric, this exercise rug is designed to draw moisture away from the body during the exercise, keeping the hair and skin dry.
Wraps around the saddle to provide coverage across the back and quarters.
Use during ridden work, to warm up/cool down the muscles effectively and prevent injury.
Fastens over the wither for easy use.
Nylon covered front section prevent wear and tear from stirrup leathers.
Machine washable.