Six Strategic Enemies

Not to be confused with Enemy Strategos Six

thinly veiled excuse to plug Ava's Demon goes here

Anyway I was thinking there's some design space for monsters that have long term strategic effects if improperly engaged outside of maiming the PCs. Things like Shriekers are a classic example- if you annoy them, they can turn the dungeoncrawl from 'don't wake the sleeping monsters' to 'avoid the patrolling monsters.' So the goal here is less 'how does this monster affect the encounter with the monster' and more 'how does this monster affect how the players approach the dungeon'

1. Long LONG Worm
stats as- rock walls with a purple worm bite attack on the head, wherever it is
A worm, 10 feet in diameter and like a mile long, dull segments of stone-plated flesh wiggling through hallways, blocking them off entirely. It moves incredibly slowly, and is nigh-invulnerable to harm- (like a Purple Worm+Stone Golem resistances) It idly pursues meat and magic and fine metal, inching forward a meter a minute. The concern is not that it will catch you, but that its slow, stupid advance will block off key halls and trap you in a dead end with something you don't want to be trapped with. It can chew through doors  and walls, and advances 1 room or hallway each exploration turn.
If somehow split, you see each segment has teeth and a digestive tract and some semblance of a brain, and then you have two worms. Would work as either practically infinite length to make the dungeon ever more worm-clogged, or just a few hundred feet long so its movement eventually frees up space behind it.

2. Animators
Stats as- zombies, animated objects
Vengeful spirits of people the PCs have killed that possess things. Corpses are preferred, but suits of armor or clothing will do with some stuffing, or even a table, or a length of rope. Anything that can let them wobble around the dungeon and try to kill people to take their corpses. They can't leave an object until it's destroyed, and they will remember player tactics from previous encounters and try to wear the players down, pick off the weak, and basically turn every piece of scenery into a potential 'mimic' threat. And also moan things like 'you killed me you bastard now we'll kill you.' They can be 'solved' tactically by imprisoning the possessed husks somewhere, but the spirits will work together to escape and wreak vengeance. If everybody (including PCs and hirelings) killed in a dungeon returns as an Animator, you can get a fun game of musical chairs but with corpses and killer furniture going.

3. Chimerelemental

Stats as-Elemental A swirling mass of air, earth, fire, and water, a thoroughly unnatural construction of mad sorcery or possibly just an elemental orgy the likes of which Man Was Not Meant To Know. Tends to activate an scenery-changing ability, possibly clobber people for a bit if they got fresh with it, then flee, to be encountered again as a lurking threat in another area or back to the wandering monster tables until the dungeon is a hopeless ruined mass of transmuted elements. Also provides shenanigan opportunities to players.
Extra Abilities- Can be targeted against smaller instances like a player's waterskin of course but tends towards indiscriminate, unsubtle effects in 10' cubes, or in arbitrary amounts like 'one room' or 'one pond'
Gelatinize Air- Turns the air into a clear jelly, much like water but conforming handily to the locale rather than flooding away. Good for drowning people and extinguishing fires.
Sublimate Earth- Turns one floor, ceiling, or wall into a cloud of swirling smoke that could be mistaken for stone by the hasty in dim dungeon lighting, as they stay in the dimensions of the original stone. Makes pit traps, hallways to other rooms
Phlogistize Water- Turns a body of water into a bright-burning substance that'll cook people alive in a jiffy, like magma but not dense.

4. Puffbuff-
Stats as- Ogre for big ones, goblins for little ones, patches of spores as Yellow Mold A boulder-sized ambulatory puffball mushroom, possibly rolling into people like the stone ball from Indiana Jones, or possibly walking around on little legs and punching people like Dark Souls, or possibly floating around like Gas Spores. Either way, killing it causes it to explode into toxic spores that deal 1d6 damage per round you're in them, and the spores block off entire rooms as 'no go poison spore' areas. The spores infest suitable things like corpses, rations, and sofas and grow new Puffbuffs in an hour or so to repeat the process.

5. Pandora's Ox

haha reusing almost related doodles

Stats as angry bovine, but with the powers & HD of all the souls they've grazed on
A evil, rune-scarred quadruped beast of burden, obviously mutated and berserk. Survives in dungeons by devouring curses and sins instead of grass, is now bloated with corrupt energies. They are vile beasts that will trample you soon as look at you, but killing them releases their stored evil energy into the environment. NOT killing them allows them to eat the souls of other wicked monsters and become wickeder themselves.
Some possible effects of released corruption, but really it should be lore and locale appropriate

• Monsters have max hp per hd, deal max damage on hits, or always act as the worst reaction results. Boring but menacing. Have them glow all redblack hell-like
• Monsters gain additional nasty powers, such as invisibility, or giant rat watchdogs, or a long tentacle, or the power to kill a yak from 200 yards with mind bullets, or dark smoke breath
• Dungeon reveals new hazards, like flooding water/lava/gas/collapsed rocks, irate scorpions, portals to hell, dormant magical trap glyphs activating

6. Lockbug
Stats as low-threat insects
For once you can blame thieves instead of wizards for training these large beetles to use their horns as lockpicks to lock and unlock doors, and to re-arm traps, and even to pick the occasional pocket. These vermin are nigh-innumerable and lurking in the cracks in the walls, so killing them only wards them away from getting up to no good until you leave the area, so the trick is figuring out how to ward the beetles away from things you don't want them tinkering with.
What you don't discover until later is that they've also been trained to collect shiny things and will be going after treasure once you humans clear a path to it, and perhaps even before.