I use good old OFF!! or the Deep woods off. OR whatever human mosquito repellent is on sale. I go out just before sunset and heavily spray their legs and anywhere else that is not covered by their sheets and then I will spay their sheets. You can also put a good fan that puts out alot of air. Or use a misting system or both. They don't like the forced air from the fan and the horses quickly learn to stand right in front of their fans. Some people have luck with Apple Cider vinegar but you have to get them to eat of drink enough. Others use Garlic but you have to be careful because it will make the animal anemic if given to much over a period of time.

Out of everything I have mentioned the OFF and fans seem to work the best. As far as your horse if she just seems to itch herself crazy ask you vet for an antihistemine to help her have less of a rection to the bites.

Hope this helps