Encyclopedia Entry:
A beastman type monster that has a small body build and the characteristics of a rat. Likes dark and moist places, and usually found in dungeons. Their size is no large than the average human child, and while they aren’t strong themselves, they are very nimble, and a pack of them can bring down a prey several times their size. They are monsters who can never remain still and are usually found running around in circles for no reason at all.
They occasionally appear in human villages to steal food and to mate, as they can only do it with a human male. The large mouse has a peculiar trait, where they will bring the human male back to their den, surround him, and take turns mating with him. In the event that there are too many of them and too few men, the younger large mice will form a new group and hunt for men in the village.
Also, as rats are usually used as targets for magic practices, these large mice have developed the ability to store powerful magic in their bodies. While they can’t cast any spells, the magics that have been stored in their bodies can be used as poison, which is transferred via biting. In the case of a human male victim, or a grown up woman, the magics will diffuse inside their bodies after they are bit. However, for young human girls, they will get a fever before fainting, while at the same time her body will undergo physical changes by itself. By the time she’s awake, she will have already become a large mouse.
In summary, the large mouse is a species that works in groups and places mating as their top priority.