We took in a young German shepard that turned out to be an excellent guard dog. He was almost dead from starvation when he showed up. We put up notices and got him back to health. One couple came to see if it was their dog. It wasn't, but they said if an owner didn't show up they would like him.
We kept him for a month or so. Even though he was less than a year old, it was obvious he was too dominant and aggressive for our place. We called the people who expressed interest and told them what he was like. They wanted a guard dog as they were away from their place all day (the wife drove a school bus).
When they came to get him he wouldn't let them up the sidewalk until I told him it was OK.
The lady called me about a year later to say how well he was working out and how he was such a good guard dog.
She related a funny story. They had arrranged for a plumber to do some work and told him to let them know when he was coming. The guy didn't and just showed up at their acreage. He started for the house when the dog saw him. All the guy could do was to make it into the school bus, which was sitting there from the morning run. The dog kept him in the school bus until the lady returned a couple of hours later. The dog did not bother the guy as long as he stayed on the bus, but threatened him if he tried to get off.
Sadly, a few years later the lady called again one spring. The dog had fallen through the ice on the pond and drowned. She just called to let us know and thank us again for the wonderful dog.