Michaela Maurice is a little closer to the answer, but I thought I would
open my big yap and voice what my Anatomy and Physiology teachers say.
 
According to what I was taught, the reason the fear poop is present in all
warm blooded life (mammalian at least) is the matter of protection.  In a
fear situation, whatever organism we are talking about is in the grip of the
FIGHT OR FLIGHT reflex... either get the he** out of there, or prepare for
battle, essentially.  Assuming that fleeing is not possible, and the critter
is now in a fight for her/his life, now we look at the primary strike
points.  The best target in a fight is the face, throat, and SOFT UNDERBELLY
of whatever you are fighting.  Face to blind, throat to cut the major
vessels... and underbelly to disembowel.
 
If we assume that for our example, a ferret is in a fight with a cat or
something, instinctively it voids its bowels in order to get the bacteria
laden foecal material out of the way.  This prevents the material from being
introduced into the abdominal cavity in the event of an abdominal wound
severe enough to open a loop of intestine or two, but not severe enough to
disembowel and thereby render the ferret helpless.  Peritonitis is the
inflammation of the perineum, the lining that holds the intestines in place.
It is almost always fatal, even in the modern day.  That is why 'gut wounds'
in knife fights, or gunshot wounds to the belly are so danged dangerous.  If
peritonitis sets in, well... suffice it to say you are in for one ROUGH ride!
 
With no foecal material present to be introduced to the abdominal cavity,
the chances are very much more in your favor of not getting peritonitis.
As I understand it, that is why even humans 'fear poop'... to protect
ourselves.
 
Just my .02 worth... of course, in the devalued currency of today... who
knows what it is REALLY worth?  :)
 
    Cheers!
 
    Bryan Hall
[Posted in FML issue 1724]