I second either the wasp spray or a water gun with some sort of stuff in there like vinegar. Since you'll be driving her, I really wouldn't suggest pepper spray. The little personal cans you get, even in the stream type, have a very limited range because there isn't a whole lot of pressure in there. So, for the first couple of uses, they are effective up to about 15 feet...if your aim is good. Every time you use them after that, the range is shorter.
Wasp spray, on the other hand, is designed to be shot over longer distances and it won't completely incapacitate you or your horse if you happen to get it on you. Pepper spray would.
If that isn't good enough, I'd suggest a bb pistol. They are not usually fatal but they would certainly give the dog something to think about.