It may be more than the fungus infecion at work here you need to get him out of the pond and straight into QT this way we can stop the spread of whatever he may have .
What worries me the most is that the fungus is covering an area of damage where the tail joines the body but if that eats it away it will join the body and infect the body if this happens you have lost the fish, even if it survives the tail will be unable to grow back.Plese have a close look and tell me what you see in detail please do the fins look red and are the ends ragged?
If so I'm guessing that this may also be bacterial possibly coloumnarise Flexibacter but it my not be it could be just as common fungal infection but as I've said the above will come true, I'm also worried about the fact its not moving.
Speed here will be of the essence where a treatmet is concered.
I not you dont have a QT facility make this your next important purchase say a 3ft aquarium with some decent filtration say a cannister filter that is man enough to cycle the tank, this Aquarium is then your QT tank and should be used only for that purpose
Buy a good fungal/antibacterial treatment and follow the instructions on the bottle to the letter
In future look closely at any fish you wish to by as well as the others in that tank, any dead or diseased fish DO NOT BUY other wise it may cost you your entire pond QTing your fish is a must never put a new fish in your pond give it a month in QT prior to it going into the pond
rgrds
Dave