This could be a sign of Equine Metabolic Syndrome. My horse has it and the first sign for him was his sheath was swollen so badly that it basically looked like he had a second sac of balls. I cleaned his sheath and checked for a bean and even had a vet check but she found nothing wrong with his penis or anything like that. I did some research and found sheath swelling to be one of the major signs along with several others such as a crest on the neck, fat around tail head, and excessive weight. I would check these on your horse and talk to your vet about it.