The Giant Rat lived in the Land of the Leprechauns.
Background[]
It is a big rat with dripping teeth and small, greedy eyes.[1] He ain't Mickey Mouse.[2]
He was able to talk and asked Graham to make him an offer and he would let him pass. Graham offered him Swiss cheese and the rat agreed to let him pass and then left.
He is likely a member of the ratkin race.
Behind the scenes[]
In the original KQ1, the rat seems to 'magically merge' with the wall[3], in the remake the rat vanishes in a puff of magic.
In KQ1AGI, it is called 'the rat' or described as the 'big rat', the 'huge rat' or the 'greedy rat'. It is called the 'Giant Rat'/'giant rat' in KQ1 Hintbook.[4] In KQ1SCI it is is the 'RAT'/Rat.
In KQ1SMS it is described as a large rat. It scuttles away.
King's Quest Chapter II has a similar creature known as a Cave Rat.