I don't have koi, but I have a comet about 3" long that is a total jerk! It was the 2nd. fish I added to our little pond, and the largest of three total. It was always the first fish to hit the food, but was otherwise well behaved. Later on we added a couple more fish, and all went well for a couple weeks. Then it was as though this largest fish went into mock spawning mode, and chased everyone. It chased one of the newest fish to the point where it stressed out and died, and then started in on the 2nd. of the new pair. Thankfully I was able to scoop the stressed fish, let it recover, and returned it to the pond. Then, NO PROBLEMS, everyone getting along again....
Yesterday I caught that comet and transferred it to our larger pond. The goldfish in that pond are all larger, and he caused no trouble among them. Then today I scooped one more fish from the little pond to add to the big pond.... Now that comet has turned into a little


again, and is chasing the tar out of the smallest fish. The same fish that he was buddies with the day before at the little pond. So now I have to keep an eye on them to make sure he doesn't kill the smallest, and to make sure all his activity doesn't stress the others.
I've never seen a goldfish act so aggressively, other than spawning. If he keeps it up he just might take the water slide to the septic tank! So, yeah, I think they have a pecking order.