Having had a huge owl TRY to get one of our ferrets to eat I can't help
but wonder if J.R. Rowling had heard of similar incidents which led to
her food choice for that critter since it is a mythological composite
which is part bird.
 
(BTW, from that experience, when being persistently mobbed by a huge owl
intent on eating your ferret the thing which works is to put the ferret
down the neck of your shirt but ONLY if you imitate the swallowing
chin-lifting motions of an owl.)
 
Oh, and we heard of an eagle taking a neighborhood cat a few years ago
from family in the Inner Mountain West (which was when we got them a
huge midwest cage for when their cats wanted to be in the backyard, as
I recall).
 
Ferrets as food seems to me to be a way to emphasize that hippogryphs
are wild creatures which live by rules other than our own.
 
I still wish it wasn't in there, but it is, it is just a very brief
mention, and the series moves on well past that.  Nor does it disparage
ferrets as I recall -- but is used to emphasize the wildness aspect of
the mythical creature.
 
Hagrid as a character does have a taste for monsters and that is
repeated over and over.  His giant spiders had a taste for Harry and
Ron.  That doesn't make JR Rowling a cannibal.
 
Just my take...
[Posted in FML issue 4539]