We have a locked gate on our river trail, so it definitely wasn't left by a fisherman. Our first thought was it was left by a cat. However, all of our cat's presents to us are left at the front or back door. Yup, I can just picture her going to the river, catching the fish, carrying it up to the house and then leaving it on the driveway. We even showed the fish to her, and she showed no interest in it. We had to nix that theory. On the other hand, there are bald eagles and a host of other birds around here who fish out of the river. There are other animals that probably get fish sometimes, but it doesn't make sense that a raccoon or fox or bear would leave it on the driveway. The bird theory seems the most plausible to us.