I caught one of these the other day and I don't imagine it was put in there on purpose.
My guess is that was someone's pet or feeder fish that they no longer wanted and decided to let go in the local pond.
To be honest I am surprised it has lived this long.
It must be a survivor.
Good on you fish.
Did I mention how I actually caught this thing? It was so close to shore I just scooped it out with my net.