Stuff like this happens all-too-often, and I get unreasonably angry at the unknown people who do it. When my dogs are with me on a trip, they are my first priority at all times. Why have one if they're not considered a part of the family vs "property"? Rhetorical question... If I had acreage I would have scooped up that little one and found him a new home.
Had a similar event happen here at my house. Someone drove into the neighborhood and dumped a little male Shih Tzu, about a year old. He was starving, scared and someone had shaved him really roughly where there were visible cuts from the shaver. We treated him, got him all his shots, did a proper groom after his hair grew back and eventually found a forever home for him with a lovely older couple in San Antonio.
They'd been looking for a dog for awhile, and were absolutely thrilled to connect with us. He's living his best life now, completely spoiled. They still check in with us from time to time to let us know how he's doing (and send a photo of him or two).