Bog Spavin is a swelling of the tibiotarsal joint of the horse's hock which, in itself, does not cause lameness. The joint becomes distended by excess synovial fluid bringing about a soft, fluctuant swelling on the front of the joint, as well as the inside and outside.
See more at Wikipedia.org...
Bog spavin chronic synovitis of the tibiotarsal joint which causes an obvious distention of the capsule of the joint. [more]Bog spavin - Community and Resources