After carefully studying and comparing your picture against known pictures, I have concluded that it is a common Goldfish. I have found that Fancy Goldfishes have a unique body shape (they have this “hump-back” look). Your Goldfish on the other hand does not have this “hump-back” look, but it does have the right body shape of a regular Goldfish; a smooth line from start to finish, like an Oscar!
Final Analysis
Body shape: Correct for a Goldfish
Size: way off for its age, but Correct for a younger Goldfish
Color: Correct for a Goldfish
Fin(s): does not contain a high colored fin(s) like most Fancy Goldfish, so it is Correct for a Goldfish
Unique body parts: Inconclusive (huge abdomen/stomach for its size)
The huge abdomen/stomach could be explained by “stunt-growth.” See, the organs are all stuff in there in a small body (like a stuff pillow), although I’ am unsure if the organs are still growing, but going by the look of your pictures, it looks like they are.
You don’t have to take my word for it, it’s only my opinion.