I believe Aiken to be correct, lockjaw (tetanus) is very common for goldfish and koi.
All you can really do is wait it out and try to treat the symptoms. if you have a quarantine tank, you'd want to get the temp as low as you can like 32-40 to slow the fishes metabolism, and increase the waters oxygenation ability, then add more air and circulation.
this makes it easy for the fish to breathe, and slow it's need to eat because force feeding it isn't going to be fun if at all even possible. the tetanus disease will progress over 2 weeks and takes about a month to recover, and the fish may not survive it in the end if organs get paralyzed like the heart.
If the fish makes it the two weeks, the jaw will free up again and start working, it will take another 2 weeks to recover from it.
No chemical or medical treatments that I am aware of do anything. Best course of action is to make it as easy as possible for the fish to ride out the condition for it's duration and overcome it. if their metabolism is slowed they won't eat much or move much so you need to have plenty of oxygen available in the water for them, it won't lose much weight and they can go a month plus, plus without eating in colder water conditions.
I don't know the pond water conditions, if it's that cold, keep it in the pond, and crank the pump way up so plenty of oxygen and circulation. if it's not that cold, a small quarantine tank, plenty of aeration freze some icecube trays of pond water and fill the quarantine tank from the pond. and just maintain the low temp on the small scale.
it's not contagious, it won't spread to the other fish, but it's outside and goldfish bottomfeed, if it gets a scratch or scrape, the tetanus toxin can get in, and their mouths will either lock open or lock closed and can get worse depending on how it affects the fish while it's affecting the fish. like if it paralyzes the heart or gills, it's pretty much gonna be game over.
But that's how you'd try to save it if you wanted to give it a shot. cold water, plenty of oxygen in the water so they have an easy time breathing, and time to wait it out.
Doing nothing might also work out, but might not, the extra helps a lot to swing it from less than a 50/50 to odds being in your favor of the fish surviving it.
