I like coming up with new monsters, especially those that challenge the expectations of the players so that they never forget that they are interlopers in a mysterious and dangerous world.
Catchers (in the Rye)
Scarecrow-husk monstrosities/aberrations created as part to catch threshers and plows as some sick druid’s attempt to stop the killing of farm plants during a harvest. The druid supposedly died (or did she) at the hands of her own creations. Now they hunt in fields, using the high grasses as camouflage and cover. Take care with your weapon, as their twisting vines and husks twist deviously to attempt to snag any weapon taken up against them. They take up residence in a field, and kill anything that comes for a harvest.
Midas Mites and Gold-Mask Scarabs
These tiny, golden hued mites (or in desert regions, scarabs) burrow into a living creature and eat it from the inside out over the course of a few days. Their bites bring about a euphoric, total paralysis and their waste, gold, is extruded through the skin of their host. By the time they are done, all that remains is a hollow golden statue of their victim filled with bones, eggs, and dormant insects. If broken, the fragile statue swarms with previously dormant mites and many eggs that are looking for new, living flesh to devour. These insects are favored for protecting ancient tombs, as any would be grave robber will soon find themselves just another golden statue to add to the wealth of the tomb. Sometimes deviously used by liches and embalmers, these insects will adapt to their undead hosts by leaving necrotic tissue in tact, and only requiring an initial investment of living tissue. Take special care of tombs filled with golden statues or upon encountering a lich/mummy with a golden mask or arms.
These oozes are congealed masses formed of the runoff of malicious and disgusting experiments. Some posit that they only want to return to a warm body. The ooze will try to engulf and force itself into the pores of any warm bodied creature it detects. What makes these oozes difficult to fight is that they regenerate too quickly to sucumb to any mortal wounds, and each attack they suffer causes more of the blood to coagulate and harden. Each hit upon the ooze gives it a thicker, scabby skin that is resistant to attack. After too many blows, the ooze becomes completely immobile.
A desiccated, rot infested corpse shambles about in the middle of the room. It moves incredibly slowly,. oozing black blood from many blows. Bodies lay strewn about it with similar battle wounds. A corpse mirror can do almost no harm on its own, but relies on foul magic to return the wounds it suffers back to its attackers with a raspy, necrotic shout. A keen observer will notice the wounds on the bodies of the fallen creatures match wounds on the corpse mirror. This undead is only harmed by those wounds which it is unable return back to its foes. Undead such as these are made either by powerful, well versed necromancers, or by the sheer rage born of a murder victim who is killed by the thing that they envied to the point of hatred. Now they seek to reflect back to the world the hate that so filled their souls during life.
Oozey amoebas of unknown origin, they tend to lurk in wet, outdoor places. Breathers appear as a wet shine on whatever surface they are hiding on, but are given away by the raspy sound of a person who can barely breathe. They curl up and spring, hoping to engulf the mouths of an air breathing creature. On engulfing a foe, they try to suck out all air, swelling up and dealing half their foe’s remaining health in damage (round down). On the third breath from the same foe, the Breather will burst into tiny gas-bladder jelly-fish, and a musky cloud that will slowly heal living creatures for one minute. Attacking a Breather will cause them to release any built up air in a poisonous defensive cloud. Rumors hint that giving air to a Breather might fight aging.
A large number of humanoids fighting as a single formation, while being led by a small cadre of captains, regiments act similar to a swarm of insects. Regiments can serve as an effective way to represent large number of weaker combatants, treating the regiment as having a single health pool, and having them scatter from poor morale once their collective health pool drops too low. Just like swarms, they are resistant to single target attacks (kill one and two more step up for vengeance) but take extra damage from area attacks due to their tight formation. Unlike swarms however, they are intelligent, armed, and take advantage of interesting things like magic users offering Regiment-wide spells, ranged attacks unavoidably blanketing an area with projectiles, and siege weapons. Players looking for advantage can even attempt to identify and eliminate leadership, with the hopes that their death will crumble the Regiment’s morale. Treating large groups of soldiers offers a way to deal with larger combat with armies, with the ease of automatic damage that swarms offer with the versatility that armies generate.