I saw a case of Clover Poison, which caused photosensitivity in the horse. It was a grey Arab cross pony. Pony was fortunate to have the black Arab skin but her two front legs had short white socks (pink skin underneath hair) that exuded a serum liquid that took the hair off. So legs were pretty ugly. Pony Club kid whom we advised to put zinc oxide on the legs, only turn out at night. Told kid she could not attend Rally with pony legs bald! Sounds mean but kid needed to care for that NICE pony!! Kid was a bit lazy but did love going to Rally, so she then provided the attention, application of skin protection DAILY, needed to clear up the problem. Pony had hair, no seepage on either leg when Rally time came around. She competed well and pony looked great.
Certain plants Alsike Clover, St Johns Wort, wild Parsnip, can cause that photosensitivity in a horse with white markings, pink skin under hair.