Interesting story Allen.
There's a couple of man made lakes not far from me. They have a good population of bluegill, LMB and catfish. In the fall they are stocked with trout
Story goes that a lady had a koi pond in her back yard. She had 2 large koi that she had had for a long time. Her husband get drunk one day, they got in a fight and he netted one and then the other and threw them in the lake. I haven't seen them in a few years but we used to catch glimpses of them cruising around the lake. They looked to be at least 3 feet long and very colorful. She offered a sizeable reward for their safe capture.
Maybe someone dropped one in your pond. Jim