"Mice" is the plural of mouse, the animal. So far there's no official parlance for the plural of mouse, the input device; the fact that "mice" is specifically a conjugation which refers to the plural of an animal with an -ouse sound (such as louse/lice) makes some grammaticians a bit worrisome about the further corruption of English, even though English is a connotative language. As such, some of them encourage the use of the kludgy and cumbersome term "mouses" instead. I personally prefer to take the cop-out approach and say "mouse input devices" so as to avoid any potential grammar vs. namespace conflict.