Apparently it was dropped off by a "portly dark-haired lady in a white truck with a small trailer" (other neighbor's mother's words). This was mid-day on Saturday. She didn't think anything of it because she's, well 80 or something and new to the neighborhood.
The guy whose pasture the horse went down in was up north until about 2PM today. His pasture connects to the property where the horse was ditched. The property where the horse was ditched was bought by a shady RE "investor" (read: *********) who tried to move his extermination business into the house until we called the county on him (he was storing all of his chems on property). Shady guy never locked up gates or anything so the person who ditched the horse had no trouble walking in, dropping her off, and bailing out.
I saw the horse Saturday evening. She was walking around fine as I drove past, and I thought, "Hm, neighbor has a new horse he's boarding". Didn't think anything of it.
Neither did anyone else until we found her covered in sweat near the road.