yikes! that looks fairly bad. I would say it looks like a straight forward ulcer and not necessarily an attack of any kind.
What is your ammonia reading? It could be ammonia burn or a parasite...but not sure until you give us all your water readings. I would start off by doing a 50% water change in your pond. What was the last water change you did? Typically, ulcers like that are caused by water quality that is not really up to par.
Separate the fish and put it into a hospital tank....then...
Go to the local drug store and buy a bottle of iodine quickly. First rub the spot with peroxide well with a thick cotton ball. If you see the ulcer bubbling, that's how you know you've done it right. Next, take your iodine and rub it onto this fish really well. It will stain the fish for a while, but I wouldn't worry about that as it will go away. Do this for 3 -5 days as iodine.
Normally, I would just say to treat your water with the appropriate salt amount next, but this ulcer is pretty bad and I would go another step. Malachite Green is an antiseptic that will help furthering of the ulcer ir bacteria getting the fish and doing worse damage. A very cheap product called QuICK Cure (any fish shop) has mala green in it and would be very helpful in your situation. Beware that malachite green is super staining (like iodine is), so you want to use it exactly the way the directions advise. You need to know how many gallons of water are in your hospital tank/pond you have so you can use the right amount. I like to use it in a hospital tank rather than using it on the whole pond, but if you don't have a hospital tank, then you have to take action.
Instead of the iodine, use quick cure for 3 days as the bottle instructs. On the 4th day, do a 50% water change, and then add the proper amount of salt to help the fish develop a strong slime coat to fight infections. Don't use the salt and the malachite at the same time. Let the malachite work on its own first, then salt.
If you choose not to treat the water, you can also paint a dab of mala green onto your fish for 5 days, but it's not the better treatment.
Oh, and if possible, when using mala green in a hospital tank don't use any tank lighting and let the fish rest in the darker setting. With a wound of that size, I would do the malachite green cure, but you can do the iodine one alone if that's what you prefer.
I hope your fish survives. Let us know what happens. Keep up with your partial water changes weekly!