The Brood Rat

The Brood Rat was given it's name for the very obvious action the mothers make after giving birth to their litter. The brood is so tightly packed that some mistake it for some kind of horror in the sewers with its wriggling back.
The Brood Rat is one who's anger and ferocity don't match it small stature. Standing at about a foot tall, these rats are vicious little beasts residing within the sewers of most cities. They have no issue attacking something many times larger than themselves. Feeding off of scraps and each other, their numbers swell when an appropriate food source is acquired. They will multiply at an alarming rate when food is found. Two or three broods have little issue in devouring a person within a day of finding the corpse. Because of this, bodies of those who have passed are cremated as soon as is allowable to ensure the Brood Rat's numbers don't swell in the area.
Records indicate that rats in the past have a pregnancy period of about 25 days. The Brood Rat however, is pregnant for only 5. A result of it's mutations over the centuries. Courting is done by the much smaller males. They must distract the female with food before impregnating her. Despite their efforts, there is a very small chance that the males will leave alive avoiding the wrath of the female.
Identifying a female from a male is done very easily as the females reach twice the size at about a foot tall with a tail in equal length.

After giving birth, the female carries her brood on her back to keep them safe from other rats and predators.