I've had two ferrets that bark.  Only occasions when they are frightened.
Both ferrets were young and slightly aggressive in certain situations.
 
Rascal was a horrible biter from an awful petstore.  When he was probably
only about 12 weeks old he tumbled down a few steps.  The bottom step was
in direct contact with a door and it was very dark.  When I reached down
to "rescue" him, he barked and attacked me.  He used to also screech at
the vacuum cleaner.
 
Years later I got Little Buster from a private breeder.  He was
unneutered.  He is a very laid back, slightly skittish boy.  Well Rascal,
was about three times his size (LB was only about 10 weeks old) used to
terrorize him.  Chasing him, scruffing him, dragging him and just down
right being hateful.  Well one day Little Bustter was running for his
life.  He suddenly spun around to face Rascal and barked at him.  Rascal
stopped dead in his tracks and just looked at him.  LB again barked and
jumped a few inches toward Rascal.  Rascal turned and ran!  That was the
last time that Rascal tried to beat up Little Buster.
 
My brother brought his dog over to the house, LB did the same thing.
When the dog got close, LB would let out a sharp bark and jump forward
a bit as if to say "Don't even think about messing with me!"
 
LB is one of the easiest going ferrets you'll ever meet.  When you pick
him up, he's like a dishrag...totally limp and passive.  However he does
have an aggressive side.
[Posted in FML issue 4504]