We recently needed a cooler after the horse got sweaty after a lesson followed by a trail ride. Thunder and a huge hail storm came in and it got really cold soon after we got back to the trailer. Our mare started to get chilled and really stiffen up - it was really scary because we couldn't walk her around safely for more than a few minutes and we had to load her up right away. Unfortunately we didn't have the nicer cooler we usually have nor did we have a sheet (we did have a lightweight one to put on). Usually those are always in the trailer box. We won't make that mistake again.
Luckily for us the mare unloaded a little stiff but much better than when she loaded and was just fine. It was probably harder on us but lesson learned.