Mus is zero-plural, so has no suffix in it's indefinite form to indicate it. Only through context, like here, can we determine it's plural (if it were singular it would be slanger spiser en mus). It is:
en mus (a mouse), musen (the mouse), flere mus (several mice), alle musene (all of the mice)
And they said slanger er ikke smukke. This alone makes them all the smukke they ever need to be!