This is a weird one. Because it’s sometimes a template and sometimes isn’t, and when it’s a template it goes by an L name but creates creatures that begin with W, but it’s a template so broad that each creature you make with it is technically three creatures, it’s not like anything else this month and arguably not like anything else in D&D. It might even be the thing this month mostly likely to be a PC. With all that going for it there was no way I was going to talk about witchknives instead.
D&D has several varieties of were-things. The most common are werewolves, but the rulebooks also have werebears, wereboars, weretigers, wererats, and occasionally dire versions of the same. Lycanthropy is usually a template that lets you rub a humanoid and an animal together until a monsters comes out, and though the resulting creature varies wildly in size, skills, and combat potential it’s universally better than the originals. Were-things don’t always lose their minds under the full moon, but they do always have DR against everything but silver, D&D’s…well, silver bullet for shapeshifters.
It’s this variability that makes them worthwhile. Need a big, strong monster who can throw wagons at the party? Combine a hill giant with a bear! Need a tricky sneak who can survive a few hits? Combine a halfling with a rat! Need a veteran of the Underdark who avoids capture by pretending to be an innocuous animal? Combine a dwarf with a giant bat! Most templates let you turn the original creature into something else, but lycanthropy can change any single creature into dozens of others. They usually have class levels to augment them further, from the obvious orcish werewolf barbarian lord to the significantly more ridiculous merfolk werefrog paladin. And you can add them to a group or adventure based on their race, their animal, their class, or anything else you want. They’re a blank slate you can use to accomplish nearly anything.
In fact, that freedom gets even broader when you strip away the lycanthropy itself and pretend you’ve made up a new creature (take a drink). The hybrid form of a lycanthrope keeps it mostly human but gives it several of the animal’s abilities. Consider the wererat, with its natural bite attack that inflicts disease. That can just as easily be a feral sewer-dwelling creature who sneaks around and ambushes lone victims. You only need to make a few changes. First, strip away the DR or change it to something more flavorful (“DR 5/attacks made in sunlight”). Second, if you don’t specifically want the monster to walk among townsfolk or pass unnoticed as a rat, take away the transforming entirely. It’s not a meaningful part of the CR, and if it doesn’t fit your flavor, don’t bother with it. Third, feel free to ignore the curse of lycanthropy it can inflict with its attacks. It’s probably safe to assume most DMs would be happier without the headache of accidentally cursing their players with combat bonuses. With those gone, there’s nothing left tying you to a specific transforming common-knowledge creature.
I’ve done this several times, especially using the 3E version of the template where you added the animal’s Hit Dice to the base creature’s. Remember when I said I’d spent forever trying to come up with a template for elite creatures? This is really, really close. I don’t always like the amount of number-crunching it takes, and I don’t always want to give tiger powers to my elite elven necromancer, but you could do worse than adding a bunch of levels onto a creature in a way that doesn’t give them overpowered monster abilities.
The only problem I see with lycanthropes, besides the bookkeeping concerns above, is that they’re all physical creatures. There aren’t any werewolf clerics or sorcerers because werewolves are about hitting things, not magic. It’s a bit of a shame, though I feel its limitations make it stronger. “Combine a humanoid and an animal” is already a task with a plethora of options. We don’t really need to make it harder by tacking on magical options too. The lycanthrope does one thing and does it well in an uncountable number of ways. That suffices.
The mystery of vampires is gone. They’re too culturally accessible. Anything that ticks off enough “obviously a vampire” boxes will send the players running for stakes and official Chosen By PelorTM prayer discs. Even the players who metagame the least can make a fair argument that their characters would know at least as much about vampires as the average television viewer does. But if you mix things up and make your own custom vampire with strange, scary, never-before-seen powers, are you actually using a vampire? Aren’t you just using a custom creature to whom you’ve attached the vampire brand? Why would you do that, except to say the word “vampire” to your players and feel smug when they assume something they have every right to assume? Vampires are defined by their weaknesses and powers. If you change their weaknesses and powers, you don’t get to say you’re using a vampire any more, and if you are using their weaknesses and powers for something else, you don’t get to be surprised or disappointed 
Instead I went the other way, using the concept of the umber hulk instead of its stats. I used that idea for bosses in two campaigns, both large insectoid creatures who lurked in shadows and had weird, confusing lairs. One passed through walls, grabbed characters, and carted them off, and a significant portion of the fight was trying to save the ranger’s animal companion from its clutches. The other was a Zelda boss, so it didn’t do much until its final battle but I peppered the dungeon with foreshadowing about it, and it was completely immune to damage from everything but environmental effects so the players had to engage it in a set piece instead. The large, crafty monster who attacks from ambush is a fine idea to strike terror in players’ hearts, and if anything the umber hulk’s stats detracted from it, so I made my own. And I got to use my huge umber hulk miniature even though the umber hulk is normally large, so that’s a bonus.
But all of those were blunt applications for a blunt object. You don’t get to take credit for innovation by using a tool as intended. Rather, the instance I’m proud of happened in the first Eight Arms campaign, which took place mostly in the party’s home city. That city had a park, and inside that park lived a druid nicknamed Crazy Eddie. He was a legal resident of the city and by law technically could not be forced to vacate the park due to outstanding statutes only he remembered. He stood against modernization and wanted to preserve the last bit of relatively natural land in and around the city. Normally nobody would care, but in Pathfinder druids could have a tyrannosaurus as their animal companion, and as it turns out a giant dinosaur is an excellent deterrent to crime and government overreach.
What followed was the most unexpected final boss I’ve ever run, and as you’ll recall from earlier in this month, I once designed a final boss during the fight that came immediately before it. The stymphalidies proved startlingly effective, blinding the party and tearing it apart with more attacks per round than was fair. Its mobility didn’t matter since the party’s main source of damage was an actual cannon, but its defensive skills worked wonders. There was a brief moment when I felt the players legitimately wondered whether they would win without casualties, and that’s half of what I want out of a final fight anyway. The other half is a fulfilling end to a story arc, and we lacked that, but if I had known where we were going I probably could have made it work too. Technically, any creature can be a climax if you sell it appropriately. I mean, your average player wouldn’t expect a chimera to be a final boss battle, but there you go.
In true rakshasa fashion, the players didn’t know they were dealing with one until the end of the campaign. He had set the wheels of the campaign in motion, convincing up a specific king to overextend himself and waiting for somebody to have a problem with it so he could set himself up as a leader once the king fell. He helped the party throughout the campaign, giving them information and connections when they needed them, and in the end he picked up the pieces. I did specifically want to avoid the “you have served me well, now you must die!” trope that always seems to knock villains off their pedestal, and once the rakshasa had power he gave the party and their guild cushy positions in his new order. I was mentally prepared for them to fight him anyway, but they shrugged and figured he was fine as long as he wasn’t actively hurting anybody (except for the paladin, who stormed off). 
Qlippoth are a race of outsiders in Pathfinder, like angels or demons. They existed before creatures who existed before time, they probably come from the most vast reaches of the Great Beyond, and they all have horrific appearance traits that inflict status ailments. Though their Bestiary 2 entry doesn’t mention the Great Old Ones, and the Bestiary 3 entry for the Great Old Ones doesn’t mention the qlippoth, their Lovecraftian influence is clear. They share no physical features and there’s no indication that they work together in any way. They have no culture besides “hurt things.” They’re D&D monsters in their purest form, bundles of dangerous abilities and little worldview or coherence, enemies of the sort only a fool would try to reason with.
Boy did I. Those pumpkin-head minis have found their way into just about every campaign I’ve run since. Sometimes they’re just an ogre with a flaming sword, but more often they’re actual pumpkins of some variety. The first time was in the Tower Campaign where each floor of the tower was its own demiplane and the whole thing was curated by a set of, essentially, DMs. We often have an abbreviated player roster during summer months in a college town, and I decided to run a gaiden, a side story concurrent with but separate from the proper plot. In it, the pumpkins were rejected creatures from a Halloween-based floor who wanted to get back at the players who no longer had to fight them. The party fought their way through six of them (well, they fought five and befriended a sixth, because my players consider any plan of mine a challenge to them and they knew forcing me to use the same miniature for both an ally and an enemy at once would cause me physical distress) before making it to the arc villain, which meant I got more leverage out of that mini than I have from most.
I still had to decide what their real-world inspiration was, and it wasn’t long before I settled on Russian. It fit geographically because I didn’t want them to share any borders with human, elven, or dwarven lands. It fit culturally given the importance of religion in historical Russia, as least as far as I researched it. It fit economically with power, wealth, and population focused more in the west, near the other countries, and less in the east. Perhaps most importantly, it was an accent I could do. With that connection we built it up more and more, keeping in mind the orcish base and the needs of our campaign setting, into the hybrid we have today.
Nerras are a race of mirror-people from the rarely-used Plane of Mirrors. They’re smooth, glasslike, almost-featureless humanoids with a ludicrous amount of elemental resistances, laughably inflated level adjustments, and spell resistance that reflects spells back on their caster. They can teleport using reflective surfaces, they have glass weapons, and basically everything they do has “mirror” in it somewhere because culture is hard. They’re supposed to be insistently neutral in alignment, but they tend toward voyeurism, kidnapping, arrogance, and sometimes massive interplanar invasions, so take that with a grain of salt.