Thanks to Jaid for sending in this illuminating link.

"Unchi-kun -- Mr. Shit. An animated coil with feet. Kind of a running gag. Seen occasionally in most volumes. His origins are given in volume 14 (Senbee had to take a "relief break" while drivingthrough the woods on the way towards town.) Unchi-kun wanders into town, is discovered by Arale, and taken to the Norimaki'shome to live in the toilet bowl there. [first case of Arale playingwith any kind of unchi occurs in volume 1.]"