The Giant Rat is one of the minor villains in the adventure video game King's Quest I: Quest for the Crown.
The Giant Rat is seen when Graham enters the cave from a hole in the ground. The Giant Rat was capable of speech and told Graham that if he wanted to get past him he needed to give him something. The cheese is the best choice because rats obviously like cheese. Once Graham gives him the cheese, the Giant Rat will disappear.
The Giant Rat will also accept the Magic Mirror or the Magic Chest but then the player will not win the game. If Graham goes near the Giant Rat, the rat will tell him to come closer. If Graham listens to him and touches him, The Giant Rat will attack him and kill him.
The Giant Rat cannot be defeated or killed in the game, so you have to give him the cheese and he will leave the you alone and disappear.