Hypothesis 1:  Trauma to the bone and/or bony joint

When the mare stepped on the foal's fetlock she fractured one or more of the bones in or 
around the fetlock.  Soft tissue trauma probably also occurred but the bone fracture is the 
main factor in the lameness.  The body responded to the fracture with vasodilation and plasma exudation, which lead to acute edema (pitting edema).   Inflammatory cells migrated to the area to the damage invoking inflammation and pain in the joint, thus 
the foal did not want to use it.