Long before my preformed pond was transformed into a life sustaining device it was a cooling off station for my labs. I put a sump pump into it pointed in the air and it basically made a fountain. This kept mosquitos out, and my dogs loved it that's all I cared about. After about 60 days like this I turned off the pump to clean the intake grill and there was a 4" crayfish in the water. We have a drainage ditch that gets water in it after several weeks of heavy rain, 80% of the time it is dry. The creek that my ditch feeds into is about half a mile away. I have no idea where this little guy came from, but he walked to my house and jumped into my pond. Soon after that of course the leopard frogs arrived, and soon after that the leopard frogs got mutliated by the crayfish. I liked him, but my wife said he was ugly, and she liked the frogs, so I moved him down to the creek half a mile away.
Anyway, stuff like this can be introduced through a little bit of carelessness on your part, but it can also show up on its own despite your every effort to keep them out.