I was talked into buying a spray for my horse for mosquitos, it is garlic based. In fact, I think they just pureed garlic and put it in the bottle.
I can't say for sure how well it keeps the skeeters off, but I sure do crave me some pasta when I use it. :(
so far the best thing I've tried is skin so soft and vinager ( but im still experimenting ) the best fly spray for horses I've tried is merigold but its really expensive! I only got it because it was on sale :P