Wolf hybrids are advertised for sale in the newspaper every day. I had no idea they were illegal here. Obviously, it's not enforced much.

There are some restrictions on aquarium fish, though the only one I know for sure is that you can't have freshwater stingrays.

How about this scenario: Someone has been breeding gerbils for awhile, but is having trouble finding homes for the offspring. They're not very smart, so they dump, say, 20 gerbils in a field somewhere. Even if less than half survive, they could sure start breeding.