The mouse is a small rodent. Often considered a pest.
In the Ancient Ones's alphabet, the mouse is represented by the letter D.
A mouse or rat lives in a hole in Dungeon Cell in Mordack's Castle.
Behind the scenes[]
The mouse at the end of KQ5 is called a mouse or rat depending on the source material. For example the automated hint line called the location of the cheese the “rat hole”. Inside the game it’s called a mousehole.
The Rat is called the Mouse in an article in Nintendo Power for whatever reason.