Sebaceous adenoma/sebaceous epithelioma?
 
Here's one:
http://www.smartgroups.com/vault/ferrethealth/Old%20YahooGroup%20Pics/
sebaceous_adenomas.jpg
 
with associated text:
 
>Two benign skin tumors of sebaceous gland origin in a ferret. Remember
>that 95% of all skin tumors are benign, regardless of how ugly they
>look. BHW. (Images contributed by Mary Mewton).
 
i have the impression that some of these grow rapidly
 
Here's a wonderful write-up:
http://www.afip.org/ferrets/Sebep/sebep.html
[Posted in FML issue 4727]