In goldfish terminology, "fancy" means double-tailed. They were first documented in the Ming Dynasty (C.E. 1368-1644) of China. From an evolution perspective, such mutation is detrimental as it makes them incapable of swimming faster and thus less competitive for natural selection. Goldfish breeders, however, do love such appearance, and various breeds has been kept ever since due to articifial selection. This design of mine was roughly based on a certain breed like the one show in the next picture.
This model was my first fish design and an example of applying my early knowledge of ERM to the 22.5° system. I still kept my old note back then, which contains my hand-drawn abstraction and the ERM map for this design, as shown below. One may compare the map with the actual map shown later and see how much they resemble each other. (In the sketch I only put one anal fin; I later realize that fancy goldfish actually have two anal fins as well, so I fixed it in the final version.)
One of the major difficulties with fish designs is that the dorsal fin tends to consume a large area due to the additional rivers in its front and rear side. This typically will cause the lakes corresponding to the body to separate far apart at least on one side, leaving huge space there that is difficult to justify. However, the doubled tail of this subject cleverly justifies the space on the rear, and it also leads to a symmetric layout that is easier to fold.