The stallion owner should have done something the minute they found out that someone put a mare in the field without their permssion.
They had an actionable claim at that time. If the stallion owner sat on this information they are in essence stating they are satisfied with the mating of these two horses. That in effect is agreeing that the foal belongs to the mare owner.
The stallion owner could have had the mare owner arrested for tresspassing, using a service (the stallion) without payment (stud fee) and potential damages if the stallion was injured.
Now if that field was not the property of the stallion owner then that is a nuther ball o' wax !