Thanks for the replies.
Yesterday I noticed ANOTHER fish with a bad abraisian / cut.
This time it COULD be caused by getting caught and rubbing against something rough. But I am leaning more and more towards a turtle. I do have turtles in an enclosure nearby, and possibly one escaped and got into the koi pond.
The new injury is a round blemish about 1/2" round, maybe a tad larger. The scales are rubbed off, and there is damage to the tissue beneath; it looks fairly deep, although there is no blood. The fish is swimming and eating and appears OK, but I'm going to try to catch him and get him into some saltwater.
It is on the flat side of the fish, so it's hard to envision a turtle getting a bite out of that area. A turtle would need an edge or corner to get a bite into. I also don't see why a turtle would go after large koi, when there are smaller, bite sized, slow moving fan tails much easier to catch.
Nonetheless, I'm worried that it's a turtle. Am planning to break the whole pond down Saturday and go huntin' for whatever is attacking my fish.