Okay, so some settings have problems with the traditional monsters. Horses get replaced with riding birds, dogs get replaced with lizards, cows get replaced with slugs.
It's all old hat, but it's very effective. You can explain it in a single sentence, because everyone knows what a giant slug looks like, and everyone knows what a cow does. And so without being threatening or confusing, the idea of spotted slugs munching on bales of hay in some bucolic oasis reinforces how alien the setting is.
You just don't want to overuse it, or else it becomes comical (Gamma World, the one normal bear in the entire world of the Last Airbender), which might be what you're going for (Gamma World is great). Otherwise, stuffing one animal into another animal's skin works great.
So here's some otters in wolf suits.
In Centerra, you won't find any wolves within 100 miles of a river. The otters all chase them away. The locals call them "rompers" or sometimes "lurkies", but really they're just dire otters. They fill all the ecological niches of wolves. They hunt deer in packs. They happily prey on lone humans. And they fill the nightmares and imaginations of villagers. They're reddish on the head, which fades to a shaggy grey on the body.
They build dams across rivers in order to make small lakes. That means more fish, and rompers like to eat fish. They also spend a lot of time playing, and will even build mud-slides down into their lakes. They live under grassy banks or inside their dams. They jump into boats to eat fishermen. That's why a lot of small boats in Asria have spiked sides, high rails, or both. They don't howl at the moon, but people have learned to fear the tchuk tchuck tchuk sound they make when they call to each other.
Warriors hunt them and wear their tails. Nobles make ornaments from their penis-bones. Villagers claim that they are shapeshifters that tempt humans to their doom. It is also said that they creep into houses at night and swish their tails over sleeping people (especially babies) which causes them to sicken and then die. A few of them have been semi-domesticated, and will chase fish into nets in exchange for most of the fish.
Romper (Dire Otter)
AC 6 
Atk Bite +2 (1d6)
And here's another one:
In the temperate areas west of the Elterspine Mountains (where you can find lots of rompers), you'll find very few of the huge, diseased rats found in other parts of the world. Instead you'll find monkeyrats.
Monkeyrats are the same size as rats (1 hp, 25% to do 1 damage with a bite). They're better climbers, and most importantly, they're smart as hell.
They're rare in the wild, but extremely common in the cities. They're smart enough to figure out ways to subsist on human excess, by stealing a couple of nuts here, eating some garbage here, and avoiding people whenever possible. They live in huge troops of up to 100 individuals, usually inside roofs or in the drier parts of the sewers.
They fight a constant, invisible war with rats, which they usually win (by virtue of cooperation) and also with cats, which they usually lose (because cats are murder machines). Still, cats in Trystero die when they get surrounded by monkeyrats, or when the monkeyrats drop a brick on them, so owners beware.
The cities have a complicated relationship with monkeyrats. They're seen as pests, but they're a bit too intelligent to treat like vermin. And they're too numerous to ignore. Huge extermination attempts have been enacted, some with partial success. But whenever the monkeyrats are killed, normal rats jump up to replace them.
And for many westerners, monkeyrats are a symbol of the people who live there, the medurans. People admire their cooperation and tenacity when fighting rats. "They're little citizens," people sometimes say. And they can be befriended, and are even intelligent enough to, say, warn their friend human when an enemy has entered the neighborhood. Urban druids rely on them extensively.
If ground up and made into a tea, their bones are believed to cure impotence in women. Likewise, their hands are frequently made into "potions"* for cleverness or quickness. It is believed that if a monkeyrat ever kills a man, the monkeyrat can become a fetus in the man's wife, and be born as a human of exceptional guile and malice.
*In Centerra, most things that are called potions aren't magical. Most potions are worthless.
And one more:
There are no vampires in Centerra. Well, there's one vampire, named St. Cascarion, and works for the Church. And I suppose there are plenty of "vampire-spawn", whose disease has given them no strengths and plenty of weaknesses (dietary restrictions and a hunger for blood). So really, when common folk talk about vampires, they're talking about sludge vampires.
Sludge vampires are sort of an intersection of a blood-hungry doppleganger and an intelligent ooze. They look like people, talk like people, think sort of like people, but under their skin is a hungry ocean of green slime.
When they go hunting, they'll remove all their clothes and slither down chimneys and under doors until they reach their prey. When they find someone asleep in their beds, they'll fling off the covers and jump on top of them. They do this because they drink blood through their skins. With enough skin-to-skin contact, they can drain of person of blood in seconds. With only a small patch of skin touching theirs, it takes much longer.
After their blood meal, the human is dead and the vampire has gained about 15 pounds in blood and incidental fluids. They distribute this weight around their body in a manner that will best disguise it depending on their size, mass, and gender, and then quietly return to their deception in the mundane circles of society.
When they wish to reproduce, they steal a baby (or buy one from the elves) and raise it as their own. When they judge that their adopted child has grown to adulthood (about 18 or so), they kill the teenager, cut off a piece of themselves (usually a foot) and sew it inside the dead person. After a week or so, the piece of slime has grown to fill the skin of the teenager, and is an adult slime. Their parent has regrown their missing hand by now, and will teach their offspring everything there is to know.
For this reason, most sludge vampires appear to be in the bloom of youth, about 18 or so. The skin that they wear always appears healthy and fresh. Because they do not age, they must often change places and identities so that people do not grow suspicious, or else shun society all together.
Like the green slime that they are closely related to, sunlight damages them. However, this only affects them if they are not protected by their skin. So if you see someone walking around in the marketplace during lunch hour and you stab them, if they bleed green stuff that gives off smoke and smells like burning, that person is a sludge vampire. If they bleed normal red blood, you've just stabbed an innocent person.
Also, their inner slime is a horrible acid. It corrodes normal metal and wood instantly, rendering it useless. You sometimes see this associated with the corpses they leave behind: drained of blood, metal objects strangely smoothed, and the scent of metal in the air.
Sludge vampires usually maintain their human facade for as long as possible, and defend themselves with "human" defenses unless sorely pressed. Sometimes they cut the skin across their palms so that they can use their pseudopod attack at a moment's notice. Savvy vampires also carry weapons inside their bodies. Quite a few are also spellcasters.
Sludge vampires can leave their skins behind. They are loathe to do this however, since they have no protection from light (even torchlight makes them unconfortable) and because they cannot maintain human form. Instead, they are just roiling oozes, like their cousins. They sometimes do this when they want to eliminate any chance of being identified on their hunts, or to keep their skins save if they expect fighting.
However, if their skin is destroyed while they are away, they cannot reclaim a new one, and will be forever stuck without higher form. Most regard their adopted skins as their "real" face, and their "real" identity, and will go to any lengths to recover them. People have even successfully blackmailed sludge vampires into obedience by holding their skins hostage.
There are three ways to identify a sludge vampire.
1) Prick them and see if they bleed. This is the least reliable method. If the sludge vampire has recently fed, they will have the victim's blood in their body that they can push through the wound, creating the illusion that they are actually bleeding. However, this is the victim's blood, not their own, which may be useful in certain sorceries.
2) Their temperature. Sludge vampires are always cool to the touch. This is a bit subjective, and sludge vampires can sometimes fool even this method of detection by swallowing hot stones.
3) Their weight. Sludge vampires gain a lot of fluid weight after they feed. If a person gains fifteen pounds overnight, they are obviously not human. Simply looking at them is not sufficient. Some vampires are clever enough to fake pregnancies, obesity, or even swallow hollow boxes to fool their observers. Of course, getting them to stand on a scale is difficult, but then, even a scale can be disguised. . .
AC 8  plus armor worn, if applicable
Atk +8 as weapon, if wearing skin
Atk +8 psuedopod, (1d8), if oozing (can attack with weapon simultaneously, if appicable)
Move 12 if wearing skin, 9 if oozing
Special weapon immunities, compressible, acid, paralytic toxin, sunlight weakness, blood drain, shape change, fragile skin.
Weapon immunities: Immune to non-magic bludgeoning and piercing weapons.
Compressible: can fit though spaces as small as 1". They can fit their skin through as well, but bulky clothing (and things like belt-buckles) will have to be left behind. They can do this extremely quickly.
Acid: all normal metal and wood that touch their slime (usually contained beneath their skin) is destroyed. Magical stuff gets a saving throw.
Paralytic toxin: their slime (usually contained beneath their skin) causes a creature to be paralyzed for 2d6 turns, exactly like an ooze
Sunlight weakness: unless protected their skin, sludge vampires take 1d6 damage each turn they are exposed to bright light. Smaller light sources (at least the size of a torch) do 1 damage each round (unless protected by their skin). Additionally, stabbing a sludge vampire with gold will do 1d6 damage every round until it is pulled out. The size of the object doesn't matter, so gold needles are often used for this purpose.
Blood drain: if the sludge vampire is touching the target (skin to skin or psuedopod to skin), they automatically do 1d8 damage on the first round, increasing by 1 point each round, up to a maximum of 8 points per round. They gain hp equal to half of the damage dealt. They can make their skin sticky, like slime, and if they surprise you, they will usually smoosh their face onto yours so that you cannot see, scream, or fight effectively. Creatures subject to blood drain are automatically subject to the paralytic toxin as long as there is a lot of skin-to-skin contact, or any pseudopod-to-skin contact.
Shape change: a sludge vampire can reshape its skin to appear as someone else. However, they can't alter their mass, and they'll always be humanoid. Sludge vampires usually keep several "wigs" taken from victims inside their bodies, so they can change hairdos at a moment's notice. This is not an illusion, but an actual change of shape.
Fragile skin: Once a sludge vampire loses more than half of its HP in a single combat, its skin is destroyed. Most are loathe to let this happen, and so may quickly slurp down the nearest sewer grate.
Rumor has it that sludge vampires come from an underground kingdom of slime, contained in a single (or possibly several) enormous cyst (or possibly a geode) deep inside the earth.