Greetings Joan:
My advice is to NOT apply anything to the bark/tree. If the bark stripping has been severe, the tree may have suffered irreparable damage...only time will tell.
You may want to install some cheap fencing around the perimeter of the tree to prevent continued browsing from the horses, but do not apply any chemical, tar, etc., as this will interfere with the tree's natural wounding processes and it may compromise your horses' health.