When horses start to lose hair, it can be indicative of a larger problem like skin fungus or allergies, so take heed.
Loss of hair across the cheeks, near the eyes, and on the front of the face is fairly common for horses especially in the warmer months, but can carry into fall as well. Some horses have dry, itchy skin, while others have slimy, crusty crud coming out of their heads. Either way, it is both unsightly for you and uncomfortable for the horse, so it is worth checking out.
Liv Gude of Pro Equine Grooms walks through some possibilities for your four legged friend’s fur-less face in this article.