Goldfish (Carassius auratus)
They might be different in shape, colour and size but all goldfish come from the same species, originating from the same parent stock. Different varieties have been developed by in-breeding selected fish but each retains a throw-back to the originals. As an example, the Celestial goldfish was bred for an Emperor who wanted a goldfish that looked up to him. Without human interference the fish will eventually revert back to their original grey colour, which helps them avoid predators. They are the most kept pet throughout the world.