Hypothesis 2-soft tissue damage-capsule tear, intersesamoidean ligament tear, I,X,V ligament tear, suspensory tear 

	Small ligaments of the fetlock joint may be torn.  This could happen from the mare stepping on the foal or the foal suffering trauma while playing in the field.  Tears in small ligaments or the joint capsule may not result in instability of the joint.  Torn ligaments cause the release of inflammatory mediators in the tissue resulting in soft tissue swelling. A small hematoma forms further separating the fibers of the ligament.  This further weakens the ligament resulting in lameness and pain. A tear of the joint capsule would result in joint effusion,  stretching of the joint capsule, nociceptor stimulation and pain.  The fetlock joint capsule extends a few centimeters up the planterolateral and planteromendial aspect of the limb above the fetlock joint.  It was not determined in the physical exam if the limb swelling was intracapsular, extracapsular or both.  This hypothesis is also consistent with no other external signs of trauma as none were mentioned in the physical.
Tendon injury occurs as a result of accute trauma or repetitive strain.
This results in inflammation.  The vascular response is capillary hemmmorhage and fibrin deposition which separates fibers and weakens the remaining tendon fibers.  The cellular response involves neutrophils and macrophages.  The function of these leukocytes is to defend area against infecton, removal of necrotic debris as well as initiating repair.