Squamous cell carcinomas (tumors that develop from skin cells) may form on a horse's penis if "beans" are not removed. Regular sheath cleaning can reduce the possibility of beans forming in the first place.
Horses are frequently sedated for a sheath cleaning, which entails an added charge on your vet bill (not to mention the fee for the cleaning itself). Many owners have worked with their male horses to make the cleaning job tolerable, and one they can do themselves.
One handy tool for cleaning the sheath is an athletic sock turned inside out. It is both soft and abrasive for thorough cleansing. The next time you're bathing your horse, rub a sponge in a circular motion near the sheath, which may encourage him to drop his penis. Regular gentle handling of this sensitive body part lets your horse know you are not going to hurt him.
Occasionally, geldings whose sheaths are in need of cleaning will rub their tails.