Gypsy, the Equine Encyclopedia, has something to say about Capped Elbow or “Shoe Boil.” Like all Shetlands, she is always watching, learning and listening.
“Capped Elbow” is a swelling of the point of the elbow which is caused by when a horse irritates the elbow bursa with a hoof when lying down. It is most commonly found in stalled horses. The swelling that results may be very large but rarely, if ever, causes lameness.
If the elbow is protected, healing is favorable. A healed wound left by a capped elbow is considered a blemish and should not impair a horse in any way.