That is not a disease or parasite, that is simply a wound that was caused by 1 of 2 things: either the fish hit its head on a sharp object in the pond like a rock or something similar or a predator tried to grab it, in my opinion.
If you don't have any rocks in your pond or things the fish can bump, its most likely a heron, bird, racoon or some other similar predator that tried to grab it and gave it a good, deep scratch in the process.
This is a fairly deep wound by looking at your excellent photos, particularly the 2nd image. You can see that the wound goes deep into the tissue. Luckily, you have no damaged scales to deal with.
Here's what I would do. Your best bet is to get that fish out of the pond and into a hospital tank to treat it until if fully heals. Unfortunately with wounds such as this, the fish survives the attack, but then is plagued by secondary infections such as bacteria and parasites that normally inhabit a pond, but healthy fish are able to fend off.
If you can't remove the fish from the pond to quarantine it, you start off by adding pond salt in the proper amounts for your gallonage. Next I would grab the fish and put it in a pan or container of shallow water. Remember that you can't keep a fish out of water for more than 30-60 seconds at a time, so if you can have enough water to just leave the wound uncovered for a few minutes, I would get a coltton ball nice and wet with peroxide and douse the wound. If you see it fizzing you know you're doing it right. If you have an iodine in the house (yep, the red stuff your mom put on your cuts and scrapes when you were a kid) and put some on that wound. It will stain the fish for a week or two, but that will go away.
If you could find
BioBandage at your local fish shop, the fish would be well-served by it, as it forms a barrier to prevent any bad stuff getting in there and promotes quick healing. It's a great product to have on hand for a situation like this.
Last, you need to net your pond to prevent future attacks and protect your fish.
Tell us if you can separate your fish into a hospital tank? Have you seen any predators? What more can you tell us?
Do you think the fish could have been startled and maybe smacked itself onto something in the pond? I've seen fish get spooked and even accidentally dash themselves into the submersible pump?
When you approach the fish after you saw this wound, did you notice if it was trying to hide and afraid? You will usually see this if a predator has attempted to grab it.
Also take note that when that wound heals it may leave a mark on the fish. Hopefully the fish will survive this and that won't matter.
I would recommend the mods to move this posting to the illness section, as the photos are very good and for future reference.
