Bog spavin is a swelling of the tibiotarsal joint of the horse's hock which, in itself, does not cause lameness.
A veterinarian will usually radiograph the hocks of the horse to check for bony changes as it is important to address the underlying cause of the joint distension.
It's important to have a veterinarian perform an equine prepurchase exam to identify an existing condition such as a bog spavin.
If the bog spavin is drained then it will simply refill unless the underlying cause has healed or been treated.
Treatment may involve injection of corticosteroids or hyaluronan into the joint and some cases may require arthroscopic surgery.