31 days, 31 monsters:
In abandoned or ruined castles, a hiraudeverie may come to dwell. Not a thing of flesh and blood, but of the symbolic. It takes the form of a heraldic beast, breaking rules apparent to the trained eye - metal on metal, for example. A hiraudeverie seeks to recreate its castle, create a mock dynasty. It passes along surfaces, stone or tapestry or skin makes no difference - animating the inanimate and charming mortals in accordance with its design. The things are rare enough that only the most learned sages refer to them as hiraudeverie. The rest know them on an individual basis, e.g. the Grambolt, the Mont-hen, the Cumbersill.
A horrid little sort of undead born from the last gurgles of someone who's drunk themself to death. They resemble blobby, jaundiced faces leaking chunky vomit from every orifice, which inhabit the drinking vessel that killed the poor bastard who spawned them like a hermit crab.
Adulthood for the tunicatemen comes with a complete metamorphosis - loss of mobility, of sentience. This can be staved off with the consumption of nervous tissue. Animal tissue only delays it. Human tissue can reverse it.
4. Human Nest
The aarakocra punish hated human enemies by force-feeding them their eggs. The human is twisted into a feral caretaker, their ribcage burst into a bony nest.
There is an asylum where the connection between flesh and soul is studied, severed from even the barest pretense of ethics. The knights-phrenologists, once prisoners there, now guard it - spikes rammed into their brains, armour bolted onto their bones. Wield gargantuan calipers with the strength to crush skulls. Accompanied by lobotomonkeys, which hump icepicks through your eye sockets.
Something like a living crystal ball. They can be scryed through, and allow the scryer to drool a proxy made of mucus into the scryed scene. For this it demands a living eye, and cares not whether the eye comes from the scryer.
7. Imp, Greathorn
An imp that manifests great pride despite its low stature will come to grow the horns of its infernal superiors - to the extent that its head is weighed down, made to drag in the dust. Delusional and blind, nonetheless has a devastating charge attack that can knock those it hits right into hell.
8. Vicaria Exilis
Maternal conduits for powers of the Upper Planes. The incarnation of such powers is a worst-case scenario, for the powers themselves and for everyone in the vicinty - forcible mass beatification, the transmutation of entire cities to salt and colourless flame, and similar events follow in their wake.
A symbiotic organism like lichen, but combining echinoderm and plant rather than algae and fungus. Each mature specimen bears a shell-encased fruit, which would require a team of miners or explosives to crack open before it's fully ripened - and the stages before it's ripened are by far the most valuable, for the ready fruit produces a lethal stench.
Horrible humanoids of unknown origin. Have something like a swallowing attack, but instead of putting you in their belly they fold you up impossibly and stuff you in the nearest container.
Also called Jumping Spiderkin. Kind of like the Germans or the Japanese, in that they're pretty chill most of the time, but when they wild out they really wild out. The female of the species is larger, stronger, and by cruel yet nigh-undeniable instinct consumed by a ravenous hunger after mating. A sufficiently-impressive display by the male can help her resist it for a time - younger and less-talented males often resort to cattle-raiding or outright kidnapping to sate their mate, igniting feuds and endearing them to no one
They don't weave full webs, but use single strands of silk ingeniously - as bungee-cords, instruments, lassos, tracking devices, and so on.
Believed to be the vestiges left behind of apprentice wizards who destroyed themselves chasing power beyond their grasp. Latch parasitically onto the similarly incautious - children, the senile, drunks, etc., letting their hosts access wild magical talents that inevitably destroy them.
A dwarf convicted of damage to property while intoxicated, failure to comply with an invitation to the wedding or funeral of a great house, cutting and/or fraying of an elder's beard, or leased work tool-related sodomy, and who was sentenced to being crushed in a hydraulic punishment press.
Near-endlessly deformable and able to hover undulatingly, retaining just about only a preternatural sense of stone and an obsession with gold and alcohol. Erroneously identified by Rubigallega the Architecton in her Commentaries on Child-Eating Things as being a Tenser's Floating Disk spell transmuted into flesh, a misidentification which has since been repeated by countless less-prominent authours.
An abnormally large and cunning sort of salamander. Its bite sucks the age out of you, eventually leaving you in a fetal state it can limply slurp up. Those who try to take advantage of the neomolynsion to maintain their youth find themselves undergoing a terrible and malforming pubescence.
Aberrant extraplanar foodies. They value rarity and difficulty of acquisition of ingredients above flavour, texture, and ethics. Bear weaponized cooking technologies to fight and trade with.
16. Summerland Corvan
Animate effigy and imperial terror-weapon. Followed by an unseasonable cold that deepens into a deadly winter. When its work is done it returns to the place of its creation, and disgorges the stolen warmth, light, and life.
17. Manus Polymitarius
Weave their lashes like a cat's cradle, into binding webs and cutting wires. Each has either a left hand or a right hand as a "head", and seeks to carve another into a matching mate.
A stalker in darkness. Invisible to any who do not have the dust of sleep about their eyes, it leaves its mark upon a sleeping victim three times across three dreams - on the third it takes possession of their body, and trafficks them to the shadow-courts and the night-hags.
Hungers for eggs, yet its poor vision leads it to swallowing many things that only vaguely resemble eggs - large gems, magic orbs, severed heads, and so on. Perhaps the remnant of a Bird Age.
20. Stillborn of the Death Gods
There now reign three gods of death - Wee Jas, Muckle Jezza, and their child Nerull - but once there were only two, and many tragedies shared between them. Being so endeared to death and the divine, these tragedies did not lie still, but quickened to the devouring of their kin and their works.
Outsiders that hail from the more chaotic planes. Must remain in constant motion, in action and in thought, or else they dissipate. Very fast, and very sharp - in just brushing their skin in passing can rasp away your flesh.
Agents of the Library of the Last Scholar. Anything they bite off is converted to information stored in their heads - only strong magic is protection against this. Some have gone rogue, and hunt down interesting stories to digest.
23. Oasis Angel
A giant insect which buries itself in arid regions, secreting a clear fluid which rises up through the earth to form false oases. Creatures that drink from such an oasis fall under the angel's spell, suffering hallucinations that led them to worship and feed victims to it.
24. Killer Hammock
In fact a sort of predatory, fast-moving sloth. Can snap out its neck to bite from quite a ways up in the trees. Fortunately unable to move beyond crawl outside of a forest.
25. Behatted Brute
Possesses a sticky, strong, and flexible membrane atop its head to which it affixes camouflage allowing it to disguise itself in an opening: as a door, filling up a hole in a forest floor, and so on. No sense of smell, yet nauseated by spicy foods.
Chthonian ectoparasite, lives affixed to the rooves of caves. Drains the energy and fat stores of prey through its proboscis, leaving them lethargic and ravenous. Its bioluminescent bulbs are used as a source of light by underground sorts who still need light to see.
Zealous ascetics who scourged themselves into macroscopic single-celled organisms. Extort the faithful to do the same. Their dogma is encoded in reams of RNA, ululating cytoplasm reciting wet prayers.
28. Hate Seed
An innovation of the weaponeers of Acheron. Suffuses a region with tarnished metal mutagens and psychic pulses of rage. A potent siege weapon in its own right.
29. Mephit, Enamel
Low-level liasions between the lords of the quasi-elemental plane of minerals and flesh. Tough, utilitarian personalities addicted to routines. Locked in an endless, genocidal war with tooth fairies.
Awful, envious beings - not as physically threatening as many other were-creatures, but their transformation allows them to infest and manipulate other people as gut-dwelling puppetmasters.
Creatures of impeccable courtesy. Become violently enraged if this courtesy is not returned. Often impose themselves as uninvited guests in well-to-do households.