Is this the type of goldfish I thought it was?

I have 3 common goldfish that I bought from Wal-Mart. My oldest one, Milkshake, turned 1 in March, and my 2nd oldest, Blackie, just turned 1. The problem is, I thought Milkshake was a plain common goldfish, but his tail fin is enormous and flowing. Is there something wrong with him? And is he really a common goldfish??