Sarazanmai

One day, three second-year middle school students — Kazuki Yasaka, Toi Kuji, and Enta Jinai — meet a kappa-like creature named Keppi, who is the self-proclaimed heir to the throne of Kappa Kingdom. Keppi forcibly takes the students' shirikodama (a mythical organ kappa steal through a person's anus) and they are transformed into kappa. Keppi informs them, "If you want to return to your former selves, you need to connect in 'that way' and you must bring me the shirikodama of zombies." At the same time, two policemen, Reo Niiboshi and Mabu Akutsu, try to do something at the police box where they are stationed.

Synopsis from Anime News Network