Answer

Your goldfish is suffering from a condition called Dropsy. Dropsy is caused by either unfavorable water conditions, a bacteria or a parasite with the latter being the least likely.
It's primary symptoms are bloating and scales sticking out.

As the fish is in a pond with other fish then
the best way to treat this is by isolating the affected fish in a quarantine/hospital tank if possible.

The next thing to do would be to add 1/8 teaspoon of epsom salt per every 5 gallons of water as the epsom salts will help draw out some of the fluid.

Change 50% of the tank water every 3 days so as to provide ideal water conditions and also try and feed some live and/or frozen foods as they are nutritionally superior to flake foods and thus will aid the fish in recuperating.

Do the water changes and epsom salt treatment for 6 days and if no improvement is seen then begin treating with a medicine called Maracyn which is an excellent anti-bacterial medication, see link below.

Please keep in mind that curing Dropsy is not easy and is an uphill battle especially if caught late.

Listed below are some links to pages that will touch upon the subject of Dropsy.

Please let me know if you have any questions.

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=4838

http://www.goldfishinfo.com/ailment1.htm#dropsy

http://www.goldfishconnection.com/articles/details.php?articleId=160&parentId=12

http://www.kokosgoldfish.com/Dropsy.html