Animals and vermin are an often under appreciated group of monsters.  Sure, the Druids get their wolf animal companions, and every party runs afoul of some worgs or a leopard or something.  Animals are ubiquitous in RPG settings.  Horses get at least low level parties from place to place, towns have an abundance of chickens and cows or whatnot, and every forest has deer to hunt, but animals are never all that scary.  Even in cinema, a place were all sorts of every day stuff gets creepy, animals really don't have a true niche for horror.  Sure, there's Alfred Hitchcock's The Birds and Cujo and...well, I guess it could be expanded a bit more if we included were beasts, but I'm not.

This movie still scares the shit out of me.

My animal of choice for Dungeon Master usage, however, is the well known, unassuming, and absolutely adorable common cat, Felis catus.  Sure, there's the whole fantasy trope of witches having black cats, and I do enjoy playing off of that, and talking cats are a big deal in certain milieus, but there's something amazing about using cats to set the mood.  If there's a cat purring by the fire, people tend to take that piece of scenery as a clue to get comfortable.  If a cat hisses at an NPC, it's an immediate tell that there's something amiss.  Feral cats in a run down area add to the feel of decrepitude, and having a cat dart off when they fail a perception check ratchets up the tension quite nicely.  My favorite use of cats, however, is as a monster.

Not like this...

One of (I believe) my most successful on the fly episodes was started by one of my players telling me that he wasn't going to be present for the upcoming session of my Pathfinder game right in the middle of what I was hoping would be an epic multi session chase.  I had to improvise.  I had been reading stuff from The SCP Foundation (which is seriously one of the most amazingly creepy things to read ever), and stumbled across the crazy cat lady article.  After freaking out completely when several feral cats started fighting on my front porch (I was reading the article at 2am), I realized what had to be done.  I pulled out some sweet templates, designed a mini dungeon, and worked out a challenge appropriate super unique monster.

The session began with the characters waking up to find their friend had disappeared.  They found his tracks leading away from the camp joined by a set of cat footprints.  They followed the tracks into a canyon, and, as they went further and further down, the rogue bringing up the back noticed that there were cats following them.  Lots of cats.  Hundreds of cats.  Their horses had run off into the canyons in a previous session, and as they tracked their friend, they came across the corpse of one of their horses.  For those of you who do not know, cats will eat larger animals.  Y'know, like people.  The horse had been stripped of most of its flesh.  A successful Knowledge (Nature) check revealed most of the teeth marks to be cats.  The horde of cats watched them make this grizzly discovery.  

Cats can be rather unnerving.

The party hastened on, eventually coming to the mouth of a cave with a small, wooden door.  Their friend's footsteps led inside, and they also realized that they were surrounded.  The only place not crawling with cats was (hopefully) inside the cave.  They entered and shut the door behind them only to be met by the stench of cat piss and a screeching, scarcely comprehensible woman.  One of the players knocked her out and killed her, and that was when the cats outside started yowling and scrabbling against the door.  Small furry bodies started thudding against the door, and claws began to scratch away the wood.  

That was pretty much the moment all the players started panicking, and once the cats scratched through the bottom of the door and started swarming them (using the D20 swarm templates to completely eff things up) they kinda fell apart.  It was a miracle they survived with the complete lack of teamwork they showed.  Two players hit other players with their burning hands sprays, and it was only someone knocking a bookcase against the door to stem the tide that got them out of deep crap.  Going out was obviously not going to happen, and so they ventured inwards.  They had a choice of two tunnels, and a deep, throbbing, spine shaking purring from one of them made them rush down the other.  

They discovered that the long tunnel they'd headed down led to a room full of sand and cat poop.  It also had a large pile of dead, decomposing stillborn kittens in the corner (yes, I went there and now have a special circle of Hell).  The far wall had a large, wooden grate, and, with the purring they'd heard earlier getting closer, they managed to knock it off its mooring enough to squeak through in time to see a giant hulking cat like form enter the room behind them.  Freaking out, they headed on.  The next room they found was a room full of metal cages which they though were empty until a pile of straw in the corner of one of them moved.  It was a dirty, scrawny, terrified, half mad humanoid. 

Whatever it was started ranting at them that the big cat monster was controlling the woman and the other cats, and he was here to breed a new one of her when she was ready to go.  He wasn't overly clear on the last part, wouldn't get out of the cage (he was terrified of the cats), and begged them to kill him.  They ended up mercy killing him and moving on.  The next room (and a dead end) they found helped clear up some of the confusion with the whole "breed a new one" where they found the preserved, mummified corpses of clones of the old woman in various stages of decomposition alongside alchemical supplies.  

After smashing stuff, they worked up the nerve to go back, afraid to spend the night to rest because they'd glimpsed several small holes that cats could get in through.  I'd also been hyping up the grossness of layers of cat fur and dander everywhere and saying that they were sneezing and their eyes were watering and that their leather armor would probably forever take on the smells of cat piss.  Y'know, environment stuff.

Basically, it was a dungeon inspired by an episode of Hoarders.

They backtracked to find the giant purring monster waiting for them.  It was a hulking mass of matted fur and rotting flesh and fangs and green glowing eyes that smelled of cat and piss and rot.  They battled it long in those caves and finally hacked it to pieces and hastened on.  Shortly thereafter, they managed to find their way out, find a holding area with their friend, and get away.  The cat swarms had dispersed with the death of their god and his priestess, and the canyon was made a little safer for a time.  And that, ladies and gentlemen, is why I love using cats in RPGs.