Some folk are literally born into darkness. Beneath the streets of Rome, Cairo, Edinburgh and most other large cities, catacombs and sewers gather waste from the lands above. Some of that waste is alive. And hungry.
Perhaps the Rat once lived in the world above, but retreated here out of desperation, fear, or poverty. He might have been born into darkness, though, growing from infancy to adulthood in a perpetual night of filth. Like as not, he's a cannibal, shit-eater or both.
Apelike Cannibals are small, weak, white and horrible, but where they exist they occur in great numbers, all bundled together like maggots. They have small clawed hands and small sharp fangs, and it by sheer numbers and persistence that they overwhelm the unwary Tourist, whom they will then eat raw. They live underground, but not very far underground, often in places like deserted farmhouses where travellers would not expect attack; though they seem to fear daylight, they will come out at dusk as soon as those in the farmhouse have relaxed. Just occasionally, they will attack a camp or a strayed Tourist in open country. They have the look of degenerate humans. Possibly they have strayed into Fantasyland via H. G. Wells' Time Machine.
These poor people know no other life, not even in their imagination. Living like troglodytes, sleeping in abandoned alcoves, these shaggy semihumanoids have never seen the sun. Clothed in tatters, they walk hunched from bone diseases and nutritional deficiencies; their skin is clammy and corpse-white. The Emperor's benevolence would extend even to such unproductive members of society, but the troglodyte specimens flee and somehow manage to elude all attempts to capture and reeducate them. Several troglodytes - unfortunately killed during capture - have been preserved and are now on display in the Galactic Museum.