It's called a mouse, I suppose because it reminds us of one. It looks similar to one, but, it is only a nickname. Actually, it is a computer input device so it could just as well be called a 'cid'. The plural of computer mouse can be either mice or mouses because it is only a nickname and therefore the plural is undefined. I prefer mouses. My nickname is Butch and if in the company of another nicknamed Butch I would prefer Butchs' to the alternative.