Smegma is the natural secretion of pasty material from the sheath. The word "smegma" comes from the Latin word for soap. A sheath "bean" is a hard, clay-like accumulation of smegma in the urethral fossa at the tip of the penis in geldings and stallions.

Beans usually do not cause problems, but in some cases, they grow very large and cause difficulty urinating and sheath swelling. Excessive smegma accumulation is more common in stalled horses, possibly because of lack of exercise and accumulation of shavings and stall debris in the sheath. You can feel a bean through the wall of the sheath as a hard mass at the tip of the penis. In a horse that has dropped its penis, it will be easier to see and feel.

Removing the bean is part of routine sheath cleaning, which you can do (see the related skill). However, routine veterinary inspection of this area is important because tumors and other disease of the sheath are relatively common. Also, vets often tranquilize horses before cleaning their sheath to relax the penis and make this process easier.