Let’s just look at Bestiary 4’s description of the ratling, shall we?
This ratlike creature has tiny human hands in place of its front paws, and an unnerving human face with a toothy mouth.
Ratlings are gross. Seriously gross, and more unnerving than even a lot of does-not-belong-in-this-reality monsters like qlippoths and hounds of Tindalos. (No surprise, since it’s based on a Lovecraft story.)
Wait, it gets better!
While they can subsist on grubs, other rodents, and carrion, they prefer living food and fresh blood, particularly that of humanoid children and elderly folk.
Feeling green yet? No?
Ratlings associate with common rats and even mate with them, producing anything from large, aggressive rats (often with vestigial humanlike features or other sickening deformities) to infant ratlings to deformed rats. In a mixed litter, the infant ratling usually kills and eats its siblings, then arranges the dismembered and disemboweled corpses in semi-occult patterns.
How about now? Also, too much hanging around near magical libraries and universities means a lot of them are divine or arcane spellcasters.
And speaking of spellcasters, a spellcaster who gains a ratling familiar gets the benefits of a commune spell! I wonder what that costs?
The master usually allows the familiar to drink her blood at least once per week.
Basically, take every creepy thing I said about rat kings yesterday, add a human face, human intelligence (Int 12), a bleeding bite, a bunch of spell-like abilities, and the likelihood that the thing is also a cultist of some dark power. (And if you think that’s bad, the original description from Pathfinder Adventure Path #49: The Brinewall Legacy is worse. How many other spellcasters mate with their familiars?)
So I’m throwing up in my mouth a little…but that also makes these excellent villains when you need a creepy/horror encounter. And not only in a standard Pathfinder game—imagine these in a Ravenoft or Gaslight England setting, or a magical academy setting à la Glantri or Hogwarts. I’m not shy about my love for two- and three-person campaigns vs. the usual group of five, and ratlings seem custom-built for such atmospheric, role-playing-heavy setups.
Next time you need to be creepy, go big—by going Tiny.
The Mage’s Hand Academy is accepting women for the first time. With no dormitories built, the decision is made to house the female students in hastily crafted rooms in the cellar of the dormitory. This displaces the local conclave of ratlings, and they have retaliated by terrifying the isolated girls one by one into doing their bidding. All of them, that is, until one girl decides she is going to either find aid from some adventurers or take the ratlings on herself.
Rumors swirl about the Comtesse de Vassar—that she is a mage, that she is a vishkanya, that she is a vampire who can walk about in daylight, that she once was a pirate queen, that she has been raised from the dead twice already. What is certain is that her ratling familiar’s face looks identical—well, aside from the razor-sharp teeth—to that of her late husband.
Children are disappearing. A local priest tells the adventurers to expect an attic whisperer, and they soon find one—or rather the remains of one, for it looks as if it had been torn apart by dozens of rats. Then the priest himself disappears…only to turn up later as a pile of bones arranged in eldritch patterns. After many blind allies and dead ends, the adventurers come across a conclave of ratlings just as they manage to complete their first summoning. Their prize is an Aklo-speaking creature that resembles an owb but hails from a dimension far stranger than the Shadow Plane…
—Pathfinder Adventure Path #49 86–76 & Pathfinder Bestiary 4 226