I had a 115 pound Lab that figured I had thrown something in the water for him to retriev, in reality it was a 60 lb beaver who slapped the water, the beaver had my lab chasing him all over this pond, lab wasn't fast enough to catch him but I doubt the beaver would attack but any animal would defend himself if they where attacked. I told a trapper about the big beaver and he took 4 out of the pond during trapping season
Now a river otter would be a different story they can get fairly big and are predators so I would not permit my dog to go after one
