My daughter brought home 2 of these little(2") guys in a cup from a school outing to a conservation area(park) a year or so ago -- and dumped them in our pond which just had Koi and 'gold' fish.
We now have about 12 of these guys. I don't know if there has been some cross-breeding with the Koi.
At the rate these things are reproducing, they may very well become bait. Or, if the apocalypse comes -- food.
Given the answers so far (carp), and comparing them to the Koi, which are a form of Carp, I think that what ever species they might have been, they are now mostly Carp/Koi.