Hypothesis 2
Since the foal seems healthy (nursing, normal temperature, bright, and alert) and the 
appearance of signs is acute, trauma seems well fitting to the facts of the case.  The mare 
probably did step on the foalÕs leg, or some other traumatic incident occurred, causing 
strictly soft tissue damage (no bony fracture here) causing soft tissue swelling.  The 
pressure from the weight of the mare on the foalÕs limb caused damage to the underlying 
tendons or tendon sheaths without breaking the skin.  The tendon may be torn or 
ruptured.  Because the foal is toe-touching, any ligaments on the plantar side of the 
fetlock (superficial and deep digital flexors, suspensory ligament), are probably intact.  If 
the common digital extensor or any other extensor ligaments were damaged in some way, 
the foal would be less able to position the hoof to plant it firmly on the ground.  
Collateral ligaments of the fetlock joint and any tendon sheaths are also possibly 
involved.
The damage from the trauma has resulted in sever tendonitis, causing the visible soft 
tissue inflammation and non-weight bearing lameness.