My goldfish moi, has been living in our backyard pond for the summer and recently develop this sore?
Picture: https://www.dropbox.com/s/dbrz8dyst5v00vc/20140812_114557.jpg
How can I get rid of it? And what is it?

Oh, he's still active and eating normally. There's another goldfish living with him as well. Along with some smaller fish like guppies.

As it's an outside fish leave it alone and see what happens.