Horses with photosensitivity do not have a sunburn. This is a more serious skin condition in horses. There are two main causes.
The ingestion of toxic plants which include St. John’s Wort, buckwheat, burr trefoil, smartweed, and perennial rye grass can cause this condition. These plants deposit photo-reactive substances in the tissues. Those particles then react to sunlight resulting in red “sun burnt” crusty skin that dies and sloughs away. Skin with less pigment (pink skin) lets in more UV rays which is why it can be mistaken for a sunburn.
The second possible cause is a complication of chronic liver disease. The liver fails to metabolize certain plant pigments which causes phylloerythrins (the end-product of chlorophyll metabolism) to be deposited in tissue which then reacts to sunlight.