The Giant Rat guarded the door into the Land of the Leprechauns beneath the Mushroom Isle in Daventry. It was a hungry rat, more than willing to eat flesh. The rat also was able to speak and could magically vanish to allow people to pass once it deemed they should. Graham fed some cheese to the rat and it disappeared so that he could enter the underground kingdom.
The Giant Rat appears in King's Quest I: Quest for the Crown.