Sunscreen can be applied wherever the horse exhibits a photosensitization reaction. You will of course avoid the eyes and the ear canal! It was designed not to melt and to be visible, it is waterproof and holds 4 days on a horse in the field that rolls and lives its life :)
It can be applied on wounds or crusts. It promotes the regrowth of hair (unlike human sun creams that contain agents to limit hair growth).
Last point, it is not toxic if the horse licks it.