The Jargon File (version 4.4.7, 29 Dec 2003):

grue
 n.

    [from archaic English verb for shudder, as with fear] The grue was
    originated in the game Zork (Dave Lebling took the name from Jack Vance's
    Dying Earth fantasies) and used in several other Infocom games as a hint
    that you should perhaps look for a lamp, torch or some type of light
    source. Wandering into a dark area would cause the game to prompt you, ?It
    is very dark. If you continue you are likely to be eaten by a grue.? If you
    failed to locate a light source within the next couple of moves this would
    indeed be the case.

    The grue, according to scholars of the Great Underground Empire, is a
    sinister, lurking presence in the dark places of the earth. Its favorite
    diet is either adventurers or enchanters, but its insatiable appetite is
    tempered by its extreme fear of light. No grues have ever been seen by the
    light of day, and only a few have been observed in their underground lairs.
    Of those who have seen grues, few have survived their fearsome jaws to tell
    the tale. Grues have sickly glowing fur, fish-mouthed faces, sharp claws
    and fangs, and an uncontrollable tendency to slaver and gurgle. They are
    certainly the most evil-tempered of all creatures; to say they are touchy
    is a dangerous understatement. ?Sour as a grue? is a common expression,
    even among grues themselves.

    All this folklore is widely known among hackers.