Koi

Koi are a species of fish (specifically carp) that live in ponds. They used to be bought as a pet in the Pet Shop. Wild Koi are not seen during the game, so it is unknown as to where the Pet Shop gets them, like the parrot.

According to Wikipedia, "Koi, or more specifically nishikigoi, (literally Brocaded Carp in Japanese), are ornamental domesticated varieties of the common carp."

Players can also see the Koi in the pond during the play Fairy Fables, their natural habitat. These Koi are quite resilient, and have a high tolerance to the cold, as a player can freeze their koi pond in their igloo and then unfreeze it and the koi will still be swimming.

Trivia

 * Koi could've been brought to Club Penguin by Rockhopper (as well as the Parrot, Goldfish, and Turtle).
 * The Koi fish can only be seen in the Koi Pond, which can be bought in the Furniture Catalog. It used to be available in the Pet Shop.
 * At the Medieval Party 2009 many Koi fish were swimming on a fountain at the Lodge Attic, you could also feed them.
 * It is unknown where Club Penguin has gotten these fish from.