Poor Old Tree

This tree is probably close to 100 years old! It is in the pasture across the street from us and one day last fall we looked out to see half the bark gone! By the next day the rest of the bark had also disappeared. I'm so sad that this happened, the horses that live in the pasture did it and I am so afraid this grand old tree is going to die. I have thought about putting paint on it to see if that would protect it, but I'm not sure the horses would leave it alone.