Tina,
If it's really cold and nasty we use New Zealand
rugs for the horses. They are not cheap but are
both waterproof and warm. They work
well but the horses can sweat a lot if
it's not really cold. If it's rainy but
not too cold then we use nylon waterproof
rain sheets for the horses (they are OK but
not great). And if it's warm enough they just
get wet.
Like you we prefer not to put them in the trailer
so they can move around in their corral and we
can try to get some sleep.
On cold or rainy morning we start out riding with rump
rugs that we have gotten from Lovell of Mack.
Stagg and Cheryl Newman
(and "The Pony and his gang).