Note that there are two plural forms given: 'mice' and 'mouses' (in the lower window). By looking at the frequencies you see that 'mice' is much more frequent. A search for 'mouses' retrieves the examples below. By examining them you see that 'mouses' is very rarely used as a plural form of 'mouse' other than in certain contexts (for example 'Mickey Mouses', spoken by children, or as an example of non-standard usage).