I would guess that it's a kind of tumor or ulcer (possibly a parasite, but it's hard to tell)... maybe take the fish out of the pond and examine him closely in a tank for a while!
Try treating the "zit" with some salt or antibiotic and see if it will get better. Watch your fish closely too in case he'd get worse! Watch how he's eating, swimming, if he's active, does he look like he's in any kind of pain?

But it's rather hard to tell you anything useful since I'm no pro on tumors, and it might be easier to figure something out if you had a picture of it.
I would still recommend taking him out and looking closely at him! Just to be sure (: You could always try borrowing some book about pond fish/goldfish and their diseases and see if you can find any symptoms like this one!