Today I really wanted to talk about the Friendship is Magic session during Delve Night, where we pitted the players against the show’s heroes, including unicorns, to disable a magical ritual that hurt anybody without friends. But on reflection, I didn’t design that session. I helped with some of it, but I really just took somebody else’s custom monsters and used them. And if I can’t talk about how I used unicorns, I have to talk about the only other U creature anybody can name.
Umber hulks are another creature proprietary to D&D. They’re something like a gorilla covered in chitin with the face of a beetle. They live underground, they burrow, and they’re pretty good at grabbing things and either eating them to death or absconding with them. Their signature ability is a gaze attack that confuses enemies, often baffling them into attacking themselves or each other. Depending on your edition they’re either somewhat clever or startlingly intelligent, and they’re better at guerilla tactics than one might expect from a cave-dwelling bug.
I just realized they use guerrilla tactics and look like gorillas. There is no need to comment about it.
I’ve always liked the look of the umber hulk, but I’ve never found a good opportunity to use them as written. My main problem is the very hit-or-miss confusing gaze. When it hits, players lose their actions bumbling around like fools, which isn’t fun for them. When it misses, the umber hulk loses most of what keeps it from being a boring claw-claw-bite monster like a dozen others at its CR, and that’s not fun for me. The things that make them fun are the things the book trusts the DM to come up with: its lair, its tactics, its priorities, and the way it moves across the battlefield. The good news is that you could take out the gaze and use the remaining stats for any burrowing creature (take a drink), but the bad news is they’re still a lot more work than other creatures are, and a lot of its fluff is also hit-or-miss depending on how the players react.
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.
I think Wizards understood some of the issues with the umber hulk given the options they presented for it. I recall one book recommending that DMs give monsters class levels to mix things up, and the example they used was an umber hulk druid. The players would approach it expecting a straightforward fight, then it would cast flame strike and the players would rethink their lives. My first thought when I read that was “that takes seven levels of druid, which isn’t a good fit for the umber hulk’s skills and abilities”, but my second was “that’s a neat idea.” The under-appreciated Enemies and Allies splatbook also had Blind Jak, a traveling umber hulk monk who wore a blindfold so he wouldn’t confuse his allies. Neither of these really works with the flavor of an umber hulk, but I appreciate it when the designers try to make a monster more interesting than it deserves.
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.
The first monstrous PC I ever saw was a minotaur, but that’s not my story to tell. I can tell you about Kulgrim of the Held Fist, the first NPC in the Unnamed Monster Campaign, where the players were all monstrous PCs. The party was tasked with killing a minotaur who had been terrorizing the locals. When they found that minotaur, he was meditating under a waterfall. After a very short period of confusion, he agreed to help the players kill the real culprits, and that set the stage for an entire campaign about being the good guys when you look like the bad guys. 
Lolth-Touched creatures are people and monsters given power by Lolth, goddess of drow and spiders and best friend to every author who couldn’t think of a more interesting villain in Faerûn. Her blessing makes them better in melee (curious for a deity whose favored race hates melee combat, but whatever), gives them a few cute but mostly trivial buffs, and makes them evil. Because it’s a template, there’s not a lot of additional story here. Lolth like thing, thing get stronger.