The traditional Shubunkin is the result of a longer goldfish breed line than those created in London.
Unlike the London breed they are not designed to be like common goldfish.
They are also not like the American Shubunkin that is designed to be very much like the comet goldfish.
Traditional Shubunkin were first created as a blood line in Japan of 1900. The creator was Mr. Akiyama.
These Carrasius auratus var (the scientific name) were the result of breeding fancy tail and globe eyes with a wild goldfish. Then selecting for the desired calico.
This would explain the heartiness of an inbred fancy fish. A smart move on the part of the breeder that knows that this type of breeding for abnormalities often produces weak genetically defective fish.
It would seem logical that before breeding for even further oddity through coupling parent fish with offspring fish; when they have the same visual trait that infusing a healthy quotient of genetic material back into the mix helps the mutation come about in a more healthy and humane manner.
For coloration variety in these fish, leaving them outdoors exposed to the sunlight is the best practice.
It has up to this point in history been harder to ship this blood line. So the resulting recreation of the breeding process occurred. This resulted in the American, Bristol, and London Shubunkin offerings.
In China these fish are called Chuwen-Chin. The Hong Kongian English term for these animals is Variegated Swallow Tail.