It is possible you saw a Nutria Rat. I saw one on the river here once that was about 4 feet long. I also ran across a Nutria that was nothing more
than a carcuss next to a well known hobo camp. It was about 5 feet long but it consisted of a head, tail, and nothing in between but some bone, in
other words carved up and ate over the campfire I discovered a few feet away. The border jumpers here will eat them when they are hungry.
www.nutria.com...
Look at this and see if it matches what you saw.