Koi appear as part of the Koi Pond and Waterfall Pond furniture items. In both items, they swim around the water of the ponds, and periodically jump out and fall back in. They can be removed from the items by pressing the up/down arrow keys.
When Fairy Fables was playing at the Stage, a koi fish would jump out of the water every so often.
During Medieval Parties from 2009 through 2012, many koi fish were swimming in a fountain at the Lodge Attic. There is a bowl on the fountain that, when clicked, pours some fish food in the water, causing several koi to swim around the food.