Well, it's rather hard to tell and some better pictures would help, as well as a little more description. Do you only have Koi or do you have comet goldfish as well? To me it looks like you most likely have a comet/Koi hybrid. We had a couple at one point. Have you ever noticed them breeding? It can be easily overlooked. They typically breed before it rains or on a warm, humid cloudy summer day. You may notice a little foam on the water's surface and see the males chase the female in attempt to get them to lay eggs. They lay a lot of eggs, however if you don't separate them or don't realize that they are spawning, most, if not all of the eggs will most likely be eaten. If there are plants in your pond, or small places for them to hide, some may survive and will come out when they are about that size. When ours spawned, we had 6 mature adults spawning so there were millions of eggs! They were everywhere! But they were all gone within a few hours. Of course our comets helped as well. Hybrids typically will present physical traits of both the goldfish and they Koi and can vary greatly in appearance and color, though usually the hybrids are very dull, usually black, brown or dark gray, sometimes with other colors present like orange on the belly that increases with age. However it isnt uncommon for hybrids to be the same color as the rest of your comets or Koi and could be very hard to distinguish. We had 4, 2 were pure black, one was black with and orange belly, and one was black with and slight purple iridescence. They grew to be about the size of most of out comets, slightly larger than ours, but still in the normal range, about 10". Hybrids are sterile and come from accidental cross fertilization, not from deliberate breeding. You said you recently added some plants to your pond? Any change in environment can stimulate the fish to breed, though one of the biggest is moving them to a bigger pond. Koi and comets typically won't over breed, meaning that if there are plenty of fish or their environment isn't very big for their size, they won't breed. Though, again, that's only typical and of course some exceptions may apply. As for the winter question, hybrids are pretty much like Koi in that aspect. As long as you have a deep enough pond (3 feet or deeper) they should be fine. Just be sure to stop feeding them once the temperature of the pond gets below 50-55 degrees because it speeds up their metabolism, making it difficult to hibernate properly. Even young fish should be fin as long as they can hibernate properly. If you don't have a waterfall, make a hole in the ice daily with a hairdryer or something. (NEVER BREAK THE ICE! it sends harmful shockwaves down to the fish) The hole will let harmful gasses escape and let in oxygen. Sorry, if I'm overloading you with info, I just wanted to cover all the basics and help answer any other questions that may come up. Of course, I could be wrong, but that is what it looks and sounds like to me. I hope I've helped. :)

I believe you are correct. They are bigger now and look very much like the fish in the link you provided. Thanks for the information. I never realized that koi and comets could create hybrids.

We may have new fish soon. It looks like the koi have spawned.