A cyst on a ferret's tail might be a chordoma.  You can look at one
here - http://www.afip.org/ferrets/chordoma.htmI [htm or html, not htmI]
I recently read about this in the FHL.  It's something that needs
immediate attention per Dr. Bruce Williams:
 
>Chordoma is a common musculoskeletal tumor which arises in the spine,
>and most often in the tail.  No, these tumors do not metastasize, and he
>will not die of it.  These tumors do eat into the bone of the veterbrae,
>and this is painful; but the good news is that amputation of the tail is
>curative.  Let's get it removed as soon as possible.
>
>With kindest regards,
>Bruce Williams, dVM"
 
Hope this helps.
 
Roary
Albuquerque, NM
[Posted in FML issue 4411]