If we're camping in a warm rain (or if the
ground is muddy from prior rain), I put a flysheet on my gelding, making it much
easier to groom and tack up in the morning.
In a cool-ish rain, he wears a
waterproof/breathable rainsheet. In
cold rain, he wears a waterproof/breathable moderately insulated
blanket.
When I add up all the sheets, blankets,
coolers, flysheet, rump rugs, etc., I think my horse has more outerwear than
me.
