Bad shape
The horse was actually tied to the lonely tree that was in the middle of a field and it was covered in blood and wounds. Not only that, but the poor animal looked like it was about to fall over from exhaustion, dehydration, malnutrition, and other things just to name a few.
Needless to say, the police officers could see that this starving horse was not in good shape and needed help right away. They wondered what had happened to it. Did another animal attack it? Maybe so, but that would not explain the way that it was tied to the tree, only a human being could do that.