Mice

Singular: Mouse/Little Mouse
Plural: Mice

1. Akin to a sidepiece/sidehoe. Someone you're casually dating, hooking up with, etc. with no or not much intent of a serious relationship. 'Bonus points' if your actual main/significant other knows about your mice/mouse.

Finds some of its roots from AHS: Hotel as a reference to the third person in the polygamous marriage of a couple in the season. She is referred to by the wife as 'little mouse'.

Person1: Is that your girlfriend/boyfriend?
Person2: Nah, just a mouse/one of my mice.