Shoggoths are large, dangerous underground dwellers (Caverns 3). They are fully amphibious and can spawn in both wet and dry underground regions. They're made of protoplasm which renders them very hard to kill and as far as offensive threat level are close to on par with FBs -- no special attacks, but they're bigger and faster. Like FBs they avoid traps and are level 2 building destroyers. Additionally, they have a brutal suite of [NOWHATEVER] tags. They lurk in the second and third layers of the caverns.
Shoggoths have a basic, 1-part body. I was tempted to leave them at that, but decided to give them some limbs/organs to attack with, as sad as it is to see them with a permanent shape. Specifically, the shoggoths have two claws, six modified tentacles that can grasp, a body eye, and a modded mouth that's rather large and attaches directly to their upper body, seeing as they have no head. Due to their similarity to Giant Sponges, they
might be invincible in normal combat.
'Protoplasm', both as a material and a tissue, had its raws copied from sponge. A few edits were made: protoplasm does not scar, is less vascular and has fewer pain receptors (Shoggoths feel no pain anyway, but in case something that CAN be hurt uses protoplasm, it's there). It can withstand heat and cold better before taking damage (but isn't magma-safe). The material also has a nonzero edge value, so Shoggoths can use the edge attack that comes with their claws (copied from Sea Monsters). Also unlike sponge, protoplasm rots and generates miasma when it does.
In 'At the Mountains of Madness', a Shoggoth is compared to a subway car -- this was used in two places. Their speed, both on land and with innate swimming, is significantly faster than a normal creature, and their volume was estimated at a 15' cube (somewhat taller/wider than a subway car, but MUCH shorter). As a result of this, adult shoggoths have a body size of 100,000,000: a full half that of the Giant Sperm Whale. Unfortunately for dwarves, they're not able to be butchered if you do somehow kill them.
Thanks to the [SOUND] tag, they can even periodically cry 'tekeli-li' for adventurers to hear
FUTURE: Shoggoths need more eyes and mouths, which will require custom bodyparts that provide body eyes and body mouths in multiple. Exactly how many they should have is unclear, which is why I've left them with one for the time being, and the claws/tentacles for more attacking capabilities.
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Dark Young are about the size of elephants, and a wee bit more difficult due to having a [GENERAL_MATERIAL_FORCE_MULTIPLIER] of 1/3. They live in any forest, including taigas. All the same, they're flesh and blood (well, ichor) unlike a lot of the more dangerous Mythos entities. They destroy buildings at level 2 but don't avoid traps. At the moment, they're not able to be tamed, and I can't think of any good reason to change this. Dark Young have the largest intended cluster size for Mythos creatures too, so they might make otherwise benign forest embarks a LOT more fun. They have modest natural skills in their attack skills, but are very good natural ambushers -- this might work on players too: their tiles are dark green ♣ for female and ♠ for male... tiles shared by ordinary trees.
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Dholes, aka Bholes (I went with dhole/dholes throughout), live at underground 5, so won't be seen normally. They secrete slime, which does nothing mechanically, and are basically giant earthworms. When I say Giant, I don't mean garden-variety giant... Dholes consumed the crust of an entire planet, and the natives couldn't stop them even with a high level of technology and magic. A minimum estimate for their size would be several hundred feet long, but I went with something... a bit more grand than that. A Dhole, at its largest (they grow over 100 years) has a size of 7000000000000... I would estimate its weight at something like 15 billion pounds, or to round 7 million tons. There's... not really a scale for that. The closest thing I have is that the US generated about 3 Dholes of waste plastic in 2010
That's what you get when you size a creature compared to a skyscraper. As if 'push' wouldn't be deadly enough with that kind of mass, I did give them an attack that uses their upper body, appropriately called 'Crush'. I haven't gotten around to testing Dholes, but I have a feeling they could pancake clowns due to the massive size disparity, and would probably be next to impossible to kill via combat (though they're not cheating like giant sponges/shoggoths). As such, I didn;t give them a material force multiplier to make them supernaturally tougher.
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Flying Polyps are Underground Depth 5 and... well, pretty much what they say on the tin: a flighted blob with a nastily large mouth. They have the hellish force multiplier of 1/10, but aren't that big: they're the same size as trolls. I made them out of the same Protoplasm as Shoggoths, since I don't want them passing through victims by having a feather-shaped tissue, as nice as it would be to have attacks pass right through the polyps without harm.
THE FUTURE: Polyps need their signature ability: controlling wind. Clearly, it's going to be an interaction or odd special attack, but I don't know what interaction I could use to have 'wind' push things around and knock them down. Material emission? cave-in style dust?
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Gugs are [LARGE_PREDATOR] (rather than [SEMIMEGABEAST]) versions of Giants. they have no noses, and their mouths and (generic-ized) teeth are of a larger than standard Relsize. They consider biting as well as striking to be a primary attack.
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Oddly, most of the functional bits of Ghasts were copied from kangaroos, because the Ghasts are described as having legs like kangaroos... at which point I found that no adjustment was made in the body detail plan for kangaroo legs -- as such, they inherited the kangaroo main kick attack, too. Their faces had the nose removed (DF doesn't track the forehead to take that away too), and they're a little bit bigger than a human. Sadly, thanks to the [LARGE_PREDATOR] tag governing the aggressive behavior of most mythos monsters, Gugs and Ghasts included, you're unlikely to see gugs hunting ghasts in the darkness in fortress mode.
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Leng Spiders are basically just above-ground GCS. Nothing much to see here.
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Zoogs are small, forest-dwelling carnivores that can be tamed as pets. If you do tame them, they'll hunt vermin and adopt owners like cats. They're slightly larger than cats and considered significantly more valuable.
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Byakhees and Nightgaunts are both humanoid-shaped fliers. Byakhees are mostly normal, but Nightgaunts have no faces and are missing most of their internal organs because of that: there's nothing for guts, lungs, or most of the digestive or excretory systems to connect to!
Both of them are tamable as exotic pets/mounts
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Shantaks are gigantic flying predators -- larger than elephants, capable of destroying buildings at the "annoying" level, and so forth. They can be tamed as exotic pets and count as exotic mounts. If tamed, they can also be trained for hunting or war