Koi Pond

The Simpsons' backyard koi pond.

A Koi Pond is a yard feature (often artificial) consisting of rocks, plants, one or more waterfalls, and koi fish that live in the pond. A traditional use of a koi pond is as a peaceful place for meditation and contemplation, enhanced by hearing the flowing water and watching the fish.

The Simpsons' koi pond

Homer bought Marge a koi pond for an anniversary present. Marge's koi pond, however, proved to be anything but peaceful, for several reasons:

When Marge sat down next to the pond, the first thought that occurred to her was to wonder how much money Homer had spent on it.
A screaming caterpillar (the Screamapillar) took up residence at the pond, making so much noise that meditation and contemplation became impossible.
When Homer decided to kill the Screamapillar to be rid of the noise ("What he needs is a visit from kindly old Dr. Foot") an EPA Scientist appeared and told the Simpsons that they couldn't kill the Screamapillar because it was an endangered species—and because it had chosen to live at their pond, they were responsible for its care.
Caring for the Screamapillar proved to be a burdensome chore for the Simpsons, as they had to read to it, burp it like a baby, and comfort it back to sleep if it woke up in the night and started screaming—which it did, several times a night.
Finally, the Simpsons got into trouble with the law when Homer thought he had accidentally killed the Screamapillar and tried to bury it, and the EPA returned in force and caught him in the act. The Screamapillar was OK, but Homer was arrested on the charges of "attempted insecticide" and "aggravated buggery".

It isn't known whether the Simpsons kept the koi pond after their issues with the Screamapillar and the EPA were resolved.^[1]

Other koi ponds

When Walt Warren moved in next door to the Simpsons, Marge and Lisa saw him carrying koi fish into the house and concluded that he must have a "water feature" in his backyard—presumably, a koi pond.^[2]