I guess it's time to get rid of the horse and put a pack of pit bulls in the back yard - then the Granby's might have something to worry about! (And they would be more than happy with "just a horse" after that.
This reminds me a bit of a newspaper article I read about a year or so ago. Lady had a small acreage that she kept her horse on. No house or anything, just an undeveloped lot if I remember right. A subdivision was built against one side of her property and the new neighbours started making various complaints about the horse and wanted it gone, even though the zoning allowed for horses and they must have seen the horse before they bought their homes.
Anyhow the newspaper article was about the pigs she'd moved in after she moved the horse out. Lots of pigs. With smelly sheds/pens built beside the neighbours fence line with automatic feeders that run day and night and are apparently loud. Apparently the lot was zoned for pigs too
Maybe these folks need pigs.