Bog spavin is a soft, fluid-filled swelling in the hock appears on the inside front or sides of the hock joint. It is caused by excess synovial fluid and/or thickened synovial tissue. The swelling usually fills in the natural depression on the front face of the horse's hock. The presence of the swelling does not cause lameness in affected horses. However regardless of the degree of lameness, the presence of bog spavin should be evaluated by a veterinarian.