While horses are generally regarded as an open range or desert animal in origin that does not mean they can survive without any relief from the summer sun. I know first hand of some of the detrimental effects that the sun has on horse left out under it with no shade for months on end.
Even with their skin covered in fur, horses can suffer from sunburn. This is more likely to affect horses with light colored skin, mostly under white hair. Sunburn appears as reddened areas and in really bad cases, blisters form, just like in humans.
One of the long term problems that horses can have is become allergic to the sun. This manifests itself as a skin problem which looks very much like a very mild case of sunburn but shows up whenever the horse is outside and exposed to light in winter as well as summer.
I have a horse that is recovering from long term sun exposure. He was kept in a round pen for 6 years with no shade in the Central Valley of California. His white skin had hair loss in patches when exposed to the sun. He was moved up north to his new home with me and we have very shaded pastures that he is now recovering in. It has taken 2 years now of the climate change from strong California sun to the milder North West sun for his skin to return to normal.
For the first year I had him, I had to liberally spray a sunflower sunscreen for horses. I also used UltraShield by Absorbine which is a fly spray that has 2 sunscreens in it. The combination of the products did the trick and enabled him to go out in pasture with the other horses. The second year I just used the fly spray. This is the beginning of his 3rd year up north and also our summer heat is just getting under way. So far so good and no more reddened skin.