The word mouse typically refers to any small rodent of the genus Mus.
There are no categorical antonyms for this word.
However, one could consider a cat, the arch-nemesis of the mouse in many literary pieces for children, as an antonym.
Alternatively, a mouse could also refer figuratively to a coward. In which case, one could consider men as an antonym for mice, as alluded to in the famous quote:
