Dormouse
This is just a specific species of mouse. The name comes from the fact that it hibernates (is “dormant”) for half the year.
Why I Looked It Up
I was reading Alice's Adventures in Wonderland and The Dormouse is one of the characters.
There was a table set out under a tree in front of the house, and the March Hare and the Hatter were having tea at it: a Dormouse was sitting between them, fast asleep, and the other two were using it as a cushion, resting their elbows on it, and talking over its head. “Very uncomfortable for the Dormouse,” thought Alice; “only, as it’s asleep, I suppose it doesn’t mind.”
(This is not to be confused with The Mouse, which is another character entirely.)
I read a book on computing history a couple of years ago called What the Dormouse Said: How the Sixties Counterculture Shaped the Personal Computer Industry. I have no idea where that title comes from.
Turns out, it’s the title of another book (which I have not read and do not own): What the Dormouse Said: Lessons for Grownups from Children’s Books.
I went back and searched the text of Alice. The phrase “what the dormouse said” doesn’t appear in the book.