Bleak Rat

The despair that fills the air of Hades is more than just an emotional burden. Many of the monsters that live in the Gray Waste perpetuate the cycle of apathy, and few embody this concept more than the bleak rats. These savage rat-like fiends are 3 feet long with gnarled gray bodies completely devoid of hair. Their eyes are narrow slits of yellow intensity – smarter than most rats in the Material Plane, yet still very low on the food chain. Enormous incisors protrude from their distended mouths.
Food for Fiends. Bleak rats are the main source of food for hordlings, diakka, and rakkix that prowl the Gray Waste. They are cannibalistic by nature, devouring each other whenever other food sources become scarce, but they always seem to bounce back in population. They are considered mere vermin by most demons and devils, but most yugoloths enjoy the savory taste of roasted bleak rat. They say the flavor is reminiscent of the hopelessness that permeates Hades and should be enjoyed with a fine goblet of aged blood wine (made from only the fattiest of mortals).
Endless Warrens. Bleak rats are found in great numbers on Oinos and Niflheim, though they are almost non-existent in the gray sands of Pluton. They create narrow warrens beneath the surface, cramped and mazelike in their intricacy, through which they hunt and feed on lesser vermin. Bleak rats do not form familial bonds but do have strong combat instincts on taking down larger prey. When they manage to bring down a larger creature, bleak rats fight each other over the choicest bits of food before gnawing on the bones in their underground warrens for weeks.

Credit to Dave Coulson from the Monsters of the Infinite Planes, available on DMsGuild