osselet

A little bone.

The internal bone, or shell, of a cuttlefish.

A hard substance growing on the inside of a horse's knee.

Such maladies as osteoarthritis of various joints (spavin in the hocks, ringbone in the pasterns, osselets in the fetlocks, navicular disease in the heels of the forefeet) are very common, as is chronic lower back soreness (coldback) in the lumbar, sacral, and thoracic regions.