I suppose it would be possible for koi and goldfish fry to get mixed together.

Do you have pictures? That would be one way to help tell. It could be a fan-tailed goldfish. Does it have the barbels around the mouth? That is a definite way to identify a koi.

May be a shubunkin?

I have several comets that look like butterfly koi until you see they have no barbels. Then the differences just start to jump out at you. They seem to be more hardy than the butterfly koi but won't ever get as large.

Yeah, what sleepless said. Comet goldfish can have long flowing fins and tail that are as beautiful as any koi, I have one such fish in my pond. My other comets have long tails/fins but his are really exceptional (yep it's a he - you can tell by who chases who at spawning time!)

It is unusual to find one mixed in with the feeders though....?

I've got all kinds of wierd babies appearing. Today I saw what must be the by-product of the black bubble eyed moor with one of the regular goldfish. Pure black, lovely fan tail and a long pointed face - ughhh! Even the tropical mollies are coming up in all shapes and sizes. No matter, love them all and their so happy not to be stuck in the aquarium any more.

Thanks everybody for the input. Maybe it is a butterfly comet goldfish. He's still a cute little bugger. Sorry about the picture quality. The fish moves so fast, this was the best I could do. Anything?

That's a koi.

LOL - hard to get a fish to pose for a picture. One of mine (the one with long fins/tail) will pause briefly and fan out his fins, showing off how handsome he is - but by the time I get the camera on him he's darting off somewhere else. How do people get such nice pics of their fish???

Yep, the whiskers would indicate koi.