Zyznian church mouse
A Zyznian church mouse was a creature known for being extremely quiet. 

In 2378, after requesting that Kathryn Janeway look after his son  for a while, Q likened the boy's quietude to that of a Zyznian church mouse, noting to Kathryn Janeway that she would "forget he's even here". ()

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de: Zyznianische Kirchenmaus
Animals