I've used blow dryers in a pinch. They work pretty good. Horses don't get that thick undercoat like a dog & if they won't burn a person they won't burn a horse.
The best bet though is to use a fitted cooler or one of those big stretchy straps around the girth area if you have a square cooler. That way you can leave the horse in a stall until dry if you don't have time to walk him/her.