I support giving the owner an ultimatum: the owner needs to build more fencing to get her away from your house, or he needs to remove her from the property. I think it is rude that they would leave a pushy animal at your property and force you to deal with it. I obviously don't know the backstory, but they made their problem into your problem.
Plus, if you're trying to sell your house, any potential buyers may be put off by a horse in their space. They may not want to even exit their cars to view the property.