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This section encompasses Jewish folklore, The Bible and The Qur'an, plus deuterocanon.
Let us begin with a preface: Nobody, with the possible exception of Hinduism and Buddhism, does the supernatural Eldritch Entity quite like the Western world's favourite religious tradition. The concept of monotheism, combined with the Platonic Theory of Forms (a direct and heavy influence on modern "Pauline" Christianity and a less direct influence on both Rabbinic Judaism and Islam) and taken to both ideas' extremes is a fantastically alien experience. All the major Abrahamic religions and apologetics emphasize how the true form of God (and all the other "higher" celestial beings, but especially God) is beyond logic, causality, physics and comprehension of the human mind, because of His Transcendent nature, and so, atheists trying to apply logic to these entities are just trying to exercise in complete futility. So, with that said let us truly start:
He was an All-in-One and One-in-All of limitless being and self — not merely a thing of one Space-Time continuum, but allied to the ultimate animating essence of existence's whole unbounded sweep — the last, utter sweep which has no confines and which outreaches fancy and mathematics alike. It was perhaps that which the religion and philosophy of Christianity of earth have referred to indirectly by the common noun God, and which has been a deity under other names; that which the Muslims worship as Allah, and by which the Rabbis of Judaism whisper by a cryptic arcane Tetragrammaton that only appears in Latin transliteration as "YHVH". Most people think it would have been pronounced "Jehovah", "Yahowah", "Yahuh", or "Yahweh", but no one can really be sure. It was theorized that properly pronouncing the name of God will only result in insanity, hence why it is a blasphemy in Ancient Jewish folklore to attempt it.
The "elder" we represent in our art is, in fact, just one of many forms and may in fact have both female and male traits. In fact God existing on such a vaster plane of existence than humanity might think of a mortal form as laughably inconvenient and beneath the dignity of His Omnipotent status. The Bible rarely even gives any description that could let one picture a physical appearance, typically calling God a spirit, and the few times it does it's made clear this is only a form He chose for that occasion. In Kings 19:12 He comes as a simple whisper in the wind but in Ezekiel 1:27...
"...[H]igh above on the throne was a figure like that of a man. I saw that from what appeared to be his waist up he looked like glowing metal, as if full of fire, and that from there down he looked like fire; and brilliant light surrounded him. Like the appearance of a rainbow in the clouds on a rainy day, so was the radiance around him. This was the appearance of the likeness of the glory of the Lord."
...and even He shows Himself in this form and speaks through it to Ezekiel His Spirit is not limited to it and continues on with other tasks.
Islam takes this bit particularly seriously; being the omnipotent, omnipresent being who created the Universe, it is forbidden to portray Him in pictures, partly, at least, because You Cannot Grasp the True Form.note It is also forbidden to portray The Prophet Muhammad or any of the other prophets, but that's because making images of them is thought to lead to eventual worship of them and that's not allowed—that would be elevating the Prophet to the same level as God, and remember that bit about thinking yourself better than the Most High? Applies when you think other people are on the same level as the Most High, too.
In fact, just looking at Him causes you to explode. The Kabbalist philosophy is that a Seraph burns by consequence of being smart enough to understand the most high's glory better than anything else. In Exodus 33, Moses had to cower behind a rock to look at the back of God!
Speaking of the Kabbalah, the demons range from angry, ravenous crows that spread like fire, to enormous, black giants covered with serpents, to disembodied, veiled heads with glimpses of horrifying eyes. The angels of sacred prostitution are much more humanoid looking though allowing them to have the Most Common Super Power.
The ancient depictions of angels are more like traditional Eldritch Abominations than the fluffy cloud angels we see nowadays on Christmas decorations. The cherubim (Yes, those cuties we all know today) have four faces - a lion on the right, an ox on the left, a human forward, and an eagle backward. They have four wings, with HANDS under the wings, their legs described as simply "straight", and they have hooves. Also, the eyes. Everywhere.Even in the spaces between the eyes. Furthermore, the exact meaning of "Cherubim" has been lost to time but upon observing a group of abnormal "Chayyot", "living creatures", Ezekiel is surprised by the revelation of them being Cherubim. This means, in the eyes of the Hebrews, their most outstanding characteristic amongst the other orders of angels was that they are alive. Not so cute anymore, is it?
Though some sources, the Book of Revelation being particularly notable, imply that there are angels so powerful and evil that God locked them up in a bottomless pit because He didn't want to bother with them. An infinite containment; even the infinite curvature of spacetime itself would probably be not enough to contain these monstrosities. And they will come out one day, at The End of the World as We Know It. The scarier implication is why God didn't use His Reality Warper abilities to eradicate these monstrosities, instead locking them up in an infinite void. Perhaps they're just as eternal as He is. The not canon to many Enoch suggests God, in a particular case of wrath (or perhaps strong parenting), filled the pit with stars to burn away their sin. But keep in mind, there are still some angels not confined to the pit that are still worrisome enough (see the revelation section below).
Some interpretations of Seraphim hold that their divine fire radiates a light so ineffably potent that it will SEAR FROM EXISTENCEanything that approaches too close, including, presumably, lesser angels. That's still nothing compared to God Himself, for whom the Seraphim are the mundane equivalent of matches, basically shining as tokens of His glory.
Seraphim are also described as dragon/serpent like (which led some experts to believe that they are loosely based on the goddess Wadjet from Egyptian Mythology, which is also a fiery snake). They consistently have six wings, and one of them is used to cover their feet. In spite of the fact that they are snake-like. Try figuring that one out. Note: "feet" is often used in biblical Hebrew as a euphemism for the genitals.
Thomist philosophy's angels aren't weird-looking because they have no appearance, being pure ideas. This concept was pretty much borrowed from Plato's Theory of Forms (Theory of Ideas). But the Platonic Ideas still definitely fit the "weird psychology" part. They have no need to reason or learn, because they know and are everything that they can know, simply as a function of their self-awareness. They are timeless and infinite and have no feelings; though they have enormous power to influence the world, it isn't by "action", as humans would understand it. The Real World as we see it is, in fact, just a shadow of these Forms, whom You Cannot Grasp the True Form. The perfect Idea of blueness "is projected" on what we "perceive" as a blue object. It's like The Matrix, only the Real World is not thisreal world but instead is mind screwy. Also, the most powerful of all angels, the supreme created entity, second only to God/The Form of the Good Himself? Satan, the Idea Of Evil.
Islam also holds the "Eldritch Abomination" view of angels. The Prophet Muhammad was terrified when he saw Gabriel in his true form; God, through Gabriel, spends the first few verses of the Sura (chapter) Al-Muddathir calming him down.
This is why every time an angel appears to a mortal, the first thing it says is always "be not afraid".
Behemoth and Leviathan are also worthy of mention. It is to the world's great benefit that these two great beasts are mortal enemies, since it is said that their offspring would be the end of the world.
In the original Jewish lore, they form a trio with the Ziz, a gryphon-like bird whose wingspan would block out the sun. The Ziz once threw away a rotten egg, and thirty cities were flooded by the liquid of it, and 300 cedars were broken by the shock of the falling egg. If the Ziz would stand in the middle of the ocean, the water would only reach his knees. Eldritch enough.
Leviathan is also said to be 300 miles long, and that is on top of breathing fire, having impenetrable scales, and glowing eyes. Behemoth is described so vaguely that all that is clear is that it is a (possibly mammalian) herbivore, incredibly strong, and invulnerable to anything that is not omnipotent (i.e. God). Not being able to describe it properly is a sure sign of an eldritch abomination.
Psalm 104 refers to the Leviathan as being an entity which God created to play with. Presumably because nothing short of an Eldritch Abomination would be any fun?
Some scholars believe Leviathan to be based on Tiamat from Babylonian Mythology, and the Hebrew creation myth supposedly goes back to a primordial battle between God and the ocean-dwelling chaos monster. Thus, the mention of Leviathan in Psalm 104 would be a Retcon emphasizing God's complete omnipotence (possibly not, Tiamat had more mammalian traits, Genesis's opening does not describe violence and Leviathon looks a lot more similar to figures actually originating out of Semite culture like Lotan but tehom, the Hebrew word for incomplete creation, resembles Tiamat)
There's also a theory that the Leviathan is a manifestation of Satan...
It gets better. Part of the prophecies concerning the End of Days is that these two beasts will kill each other... and afterwards the righteous will eat them in a great feast. Did You Just Eat Cthulhu?
One of the strangest parts of Revelation is its descriptions of Jesus. These include a man with a head and hair that is pure, snow white, eyes of fire, and feet of brass, with stars in his hands and a sword from his mouth who shines like the sun, and a lamb with a slit throat, seven horns, and seven eyes. Either Jesus becomes a Humanoid Abomination, or he's unlocked the ability to turn Super-Saiyan.
Revelation is full of them. The Beast, with its seven heads and ten horns, is one well known example.
Satan becomes one in Revelation as well; he is described as a "great red dragon" with seven heads, ten horns, and ten crowns, so powerful that a single swipe of his tail sends one-third of the stars falling out of the sky. This is from the description of the War in Heaven between Satan and Michael, each with their own army of angels. A common interpretation is that the fallen stars are the fallen angels that sided with Satan. So a third of all angels fell.
Remember, the Book of Revelation (and indeed, the entire New Testament) was originally written in Greek. The Greek word "aster" does not mean, "naturally occurring nuclear fusion reactor." Rather, it means "celestial body." "Angel," by contrast, means "messenger." Thus, "aster" is a physical description, whereas "angel" is a job description.
Though they're generally agreed to not be physical beings so much as metaphorically-described human empires, the four beasts seen by the eponymous prophet in chapter 7 of Daniel probably deserve mention here. They dominate the world one after another; the first resembling a winged lion with the heart of a human, the second a colossal bear that feeds on human flesh, the third a leopard with four heads and four wings (and in some interpretations, draws on a variety of big cats, as well), but the most eldritch of them all is the last one, which is notably not described as looking like any Earthly animal. It is described as having enormous claws, teeth made of iron, and ten horns which are the ten kings that rule the empire that the whole beast represents. In addition, the creature's smallest and most innocuous horn is strongly implied to be the most powerful and evil world leader the Earth will ever know. This thing is prophesied to singlehandedly rip the planet to shreds, and it ultimately takes God Himself to finally kill it at the end of the world.
Jewish folklore names the little known Rahab as the "demonic angel of the sea" representing the primordial abyss, the water-dragon of darkness and chaos. Often associated with the above Leviathan, Rahab was a primordial entity that God slew at the beginning of Creation. He's mentioned so sparsely in Biblical accounts that you have to know where to look to find any mention or discussion about him, but this thing was around BEFORE God got to creating the world. In other words, out of all the things in Creation, Rahab was the one thing God did not create.
Khaos, when portrayed as a deity and not a giant void that reality came from (and one of the Protogenoi, who aren't just older than the Olympians, they are older than even the Titans! If Greek myth is paralleled to Lovecraftian myth, then the Protogenoi are the Outer Gods and the Titans are the Great Old Ones), is described by Ovid as "rather a crude and undigested mass, a lifeless lump, unfashioned and unframed, of jarring seeds and justly Chaos named." Also, Chaos is the mass on which all the other gods (and therefore the cosmos, with the gods being Anthropomorphic Personifications of universal laws) emerged. A lumpy, crude, unframed mass, creating the universe as we know it? Throw some tentacles on it, put it in the center of infinity, and call it Azathoth!
The rest of the Protogenoi starts with the hermaphrodite Phanes Protogonosnote the Firstborn who was born from the Orphic Egg, which had the serpent Ophioneus wrapped around itnote Making this serpent even bigger than Leviathan, Jormungandr, and Nidhoggr!. The protogenoi also includes Gaeanote the entire planet, and her husband Ouranousnote the sky itself, when he isn't also her son, that is. Tartarus, which came inhandy when Ouranous found that the first batch of children to come from the union of two Incestuous Abominations were equally abominable, see below. Before Neptune, Oceanus, Pontus, and all the other Greek Gods and Goddesses of the sea/ocean et al, there was Thalassa; sometimes she is Gaea's daughter, sometime her granddaughter by Aethernote Light, male and Hemaranote Day, female, the counterparts of which, Erebusnote Darkness, male and Nyxnote Night, female, are also a couple.
The first children of Gaea and Ouranous were the Hecatoncheires (meaning The Hundred Handed Ones), who had one hundred arms, one hundred hands, and fifty heads with no further details of their appearance given. Compare that to Ravana◊, who had only ten heads and twenty arms, though even that is enough to put him on the border of this trope. Now try and imagine what the Hecatoncheires looked like. You can't quite do itcan you? Nope, the only name the Greeks could even come up with for these things was basically to call them "those things with one-hundred hands", which was about all we humans could comprehend of them. Well, here is what D&D◊ came up with when they asked their artists to draw one.
It is well worth noting that the Hecatoncheires were on the Greek gods' side against the Titans, throwing a hundred mountains in a single salvo apiece - much like having a living artillery piece as an ally.
Typhon, the youngest and most powerful of Gaia's offspring. Lower half consisting of serpent coils, a human upper half that reaches the stars, arms that spanned the East and the West covered with live dragon heads, a body covered in mighty wings, a main head like that of a horse and eyes that shot forth flames. When it first appeared, all of the Greek gods except Zeus & Athena ran like hell. And even Zeus, the most powerful god of them all, wielding his mighty thunderbolts in battle, lost the first round against Typhon (by Typhon STEALING ZEUS'S SINEWS and HIDING THEM) and barely managed to seal it away under Mount Aetna in round two. Before it was sealed away, Typhon also fathered most of the monsters present in Greek Mythology, such as Cerberus, the Sphinx, Orthus, the Nemean Lion, the Hydra, and the Chimera (their mother Echidna might also fit the bill). And how did Gaia give birth to this beast? By sleeping with Tartarus, a.k.a. the Greek Underworld. The Earth slept with ancient Greek Hell to give birth to a monster that frightened the gods themselves.
It gets worse. In some versions, he has a hundred "dragon" heads on his shoulders, which he can rip out to create a fully-grown monster; just one of those was enough to guard an imprisoned Zeus from the other Gods.
In some interpretations, the main head is a perfectly normal human's... that's red. And some works opt to replace all the heads with human ones. Or baby heads.
Although the fact that Zeus was willing to fight it says volumes for the guy's courage. And his re-entry into the fight was pretty awesome in its own right, too - he jumped off Olympus and started hurling thunderbolts before the inevitable end-battle finale of beating the thing over the head with an entire mountain, hitting him so hard that he sent him all the way down into Tartarus. Yes, that's right: Zeus clubbed Typhon into another world... with a mountain as his club and the other world being the Greek equivalent of Hell.There's a reason why he's King of the Gods.
A different version upgrades it from Mount Etna to the whole island of Sicily - Typhon was standing in the sea before being hit. And as mentioned above, the blow didn't kill him but only trapped him underground - the flames, smoke, and lava of the volcano Etna come directly from his mouth.
The Earth produced another one when she slept with her own grandson Poseidon, unless she is one of Ekhidnai's and Typhon's children, that is. Charybdis was apparently once a beautiful naiad, but was transformed by Zeus into a horrible and utterly inhuman monster. According to The Other Wiki, in some versions, she is a huge bladder of a creature whose face was all mouth and whose arms and legs were flippers that belches out whirlpools, while in others, she is a giant whirlpool. When forced to choose between Scylla and Charybdis, Odysseus quickly chose Scylla for good reason.
Charybdis is (after Homer, the author of the Illiad and the Odyssey) a giant whirl of water that sucks three times a day enormous amounts of water, devouring entire fleets in the process, and spits it out with a loud roar. Skylla is a monster with numerous heads which also destroys ships by eating the crews... The trick behind passing those two monsters is to slip exactly between them. Unfortunately, Odysseus gets too close to Skylla the first time, since he wants to avoid Charybdis, and loses many comrades to Skylla. Later on, his fleet is utterly destroyed by a storm and Odysseus, who's holding onto a part of a ship for his life, gets pushed back to Skylla and Charybdis and only survives to Charybdis since he manages to catch the branch of a tree on the rock hanging over Charybdis and wait for Charybdis to spit out its water to get something to float on...
According to some versions of the story of Dionysus, a god's true form will kill a mortal in front of him/her, as evidenced when Zeus appeared in all his glory in front of Semele, Dionysus' mother, who was incinerated in the process (Zeus' "true form" was apparently a powerful tempest of thunder and lightning, since they were his main powers). This happened because Hera, jealous of the affair Zeus had with Semele, tricked Semele into asking for it when Zeus swore to grant Semele anything she'd ask for, despite Zeus' pleading not to due to the aforementioned consequences.
Ýmir, a frost giant and the first living being. He was so big that the Earth was made out of his corpse (the soil from his flesh, the stones from his teeth, the grass from his stubble, etc). This means that he was at least as big as the planet.
Much larger than the Earth. His skull was used to make the sky. That means that his head alone was bigger than our whole world.
One of the monstrous offspring of Loki and a giantess, Jörmungandr started out big and grew so large that he encircles the world. He sleeps at the bottom of the ocean depths and waits for Ragnarök — not unlike Cthulhu—where he will storm Ásgarðr (in the company of his brother, Fenrir, and father, Loki), Mutual Kill Þór, and help to bring about the end of the world.
Níðhöggr chews the roots of Yggdrasill and has human corpses as snacks... To put its size in perspective, Jörmungandr may sit in an ocean, surrounding all of Miðgarðr in its coils, but that ocean still just sits on the end of one branch of the World Tree, while Níðhöggr's massive form encompasses the entire rootsystem of the World Tree itself. Also, in The Stinger of the Poetic Edda it's explicitly stated that he will survive Ragnarök.
There is also an eagle called Hræsvelgr at the top of the World Tree, and is the enemy of Níðhöggr (implying that they are of about equal size and power), and he has a hawk named Veðrfölnir sitting on his head.
Ratatoskr is the being that carries messages between Níðhöggr and Hræsvelgr. Commonly taken as a squirrel, albeit a squirrel large enough to climb up and down the World Tree with ease. Its name comes in two parts, rata- meaning climber, traveler, borer, gnawer - and -toskr - meaning tusk (or at least tooth). Thus Ratatöskr can be interpreted to mean the Climber Tusk, Bore-Tooth, Drill-Tooth, or Tusk the Traveler.
Dáinn, Dvalinn, Duneyrr, and Duraþrór are Harts or Stags, also said to live among the branches of Yggdrasill.
Set, while something of a Humanoid Abomination, is associated with an animal often called the Set Animal, or a Typhonian Beast, sharing its head. The thing is, even now, historians and archaeologists have no idea what kind of animal this might be... if it is, in fact, anything that exists in this world. Whatever it was, it was a creature that the ancient Egyptians associated with nothing less than pure evil.
Apep, or Apophis, from Egyptian Mythology was a gigantic serpent-demon that embodied chaos and darkness. Every night it tried to eat Ra as he passed through the underworld, and every night Ra killed it - but it always came back. Sometimes it was strong enough during the day to temporarily consume Ra before his attendants cut him free again (this was the Egyptian explanation for solar eclipses). Prior to the demonization of Set, Apep was the canonical ultimate evil of Egyptian Mythology, and there's no solid evidence that it was ever worshiped as a positive force. There were temples in Egypt dedicated to the worship of Apep, but it was more along the lines of praying "Please, please, please, please do not let this monstrosity ever win." It was sort of worship AGAINST Apep.
Nut/Nuit/Newet/Neuth, primordial goddess of the sky and EVERYTHING in it. She separates the heavens from the earth, gives birth to and eats the sun and the moon every day, and sometimes appears in the form of a cow.
Geb, primordial god of the earth. His laughter creates earthquakes. Usually depicted with a snakes head, or as a ram, bull, or crocodile. With Nut, he fathered many of the more famous Egyptian Gods.
Shu is the father of Nut and Geb and god of the air. Artwork and legends often have him separating his children.
Nu was the Primordial Egyptian deity of the watery abyss and was usually male, but could be the female Naunet or the male Nun (not to be confused withNuns N' RosariesNuns). Nu's repressentations varied in more that just gender, ranging from the animal (frog, snake), the humanoid (blue skinned old man), somewhere in between (frog-headed man, snake-headed woman), and the abstract (sacred lake, underground stream).
The Egyptians also believed in Ammit, which was likely the closest thing they had to a concept of Hell. She was a demon of some sort (no myth ever truly explained where she came from) described as a giant cross between a crocodile, lion, and hippopotamus, three animals that the Egyptians believed to be man-eaters. She lurked in the shadows of the Hall of Judgment, and would deliver a horrible second death to any soul whose heart was weighed down by sin on the Balance of Ma'at. (Either by devouring them entirely or just their heart, depending on the version.)
Fon Religion gives us Dan the snake, a snake that has coiled itself 3500 times above the universe and another 3500 times below it to hold existence as we know it, which it also helped its god create, together. Some Christian Fon identify Moses's staff as a representation of him. In Vodoo religion, which also has origins in Benin, Dan is identified with as one of the two serpents of Damballah, whom the large majority of spirits (including many divinities in their own right) are lesser aspects of.
Ayida-Weddo, a snake married to Damballah that simultaneously dwells in every source of fresh water, including the water in trees and is capable of altering the orbits of planets and stars. She appears to people as a rainbow.
Nyname of the Ashanti is pretty eldritch in form, the sun is merely one of his eyes. Ngula, who gave birth to him, was created by him before she did so. She is somehow both his mother and daughter. He is a very nice guy so it is hard to call him an abomination. His response to being struck repeatedly by an old woman was to not come back to Earth rather than strike back.
Amenominakanushi, who is considered a sort of Supreme Being in some forms of Shinto and considered the center of all creation; all other gods, called kami, are considered a part of him. Kami themselves could be considered Eldritch Abominations as well, due to just how vague the definition of "kami" is, with descriptions ranging from anthropomorphic to amorphous. Shinto is an animistic religion, so everything in the universe has a kami, including humans. The kami also symbolize the forces of nature, the cosmos, the laws of physics, etc. When you put that all together, that means that the supreme deity of Shinto did not create the universe; it is the universe itself! He is an Eldritch Abomination consisting entirely of millions upon zillions of lesser Eldritch Abominations, including us.
There's also the relatively obscure Japanese star-god Amatsu-Mikaboshi (literally "August Star of Heaven"), also called Ame-no-Kagaseo ("Brilliant Male"), who most likely qualifies, given the two rather... curious passages in the Nihon Shoki that mention it, both of which constantly refer to it as "the evil kami" and seem to suggest that at least three major deities - one of whom was the symbolic founder of martial arts - were required in subduing it. And that's not even getting into the moredubious sources, some of which seem to imply that the August Star is less a spirit per se than a malevolent force that predates creation.
Hundun, similar to Khaos above, is a monster formed from the remnants of the proto-world that was not shaped into the ordered universe in Chinese Mythology. Interpretations of what he is vary: he is sometimes an internal organ-less celestial dog, a faceless giant, a sentient force of nature, or even just a huge lump of flesh. All are Lovecraftian, but he is usually depicted as quite nice. In one story, two Emperors, Shu and Hu, thought that since they had holes in their bodies (eyes, mouth, nostrils, etc), Hundun should have them as well. They thus drilled those holes in Hundun, killing him in the process. In other words, Hundun was an Eldritch Abomination that got punched out by overzealous public servants.
Also in Hindu Mythology we find Krishna, in the 11th chapter of the Bhagavad-Gita, gets a description befitting any great old one: an infinite number of mouths, an uncountable number of arms holding an infinite variety of weapons, fiery breath that destroys the entire cosmos, and so on. This is the mainhero in several versions of Hinduism.
Most artistic depictions of Hindu gods are very eldritch, weird and psychedelic.
Add to that how in Hinduism, absolutely Everything, from the most powerful Gods to the smallest particle, are simply very microscopic aspects of the supreme omnipresent energy-like Primordial Entity that which cannot be exactly defined (Brahman, which is what happens if you took Yog-Sothoth and the Biblical God above and turn the "all-in-one-and-one-in-all" aspect up to even more inconceivable levels), and it all gets so Mind Screwy that you'll need some kind of a chart to sort it all out.
The Buddha, or at least the Mahayana (Buddhism merged with Far Eastern mysticism) version. Check his portrayal in Journey to the West where he is bigger than the entire universe.
Babylonian mythology gives us some frightful critters, but the (literal) mother of all of them is Tiamat. An Explosive Breeder made to symbolize the primordial sea, her exact appearance is unknown. Later depictions portray her as a dragon or serpent-type creature, but the most ancient portrayals described her form as including a tail, thighs, "lower parts" (which shake together), a belly, an udder, ribs, a neck, a head (skull, eyes, nostrils, mouth, and lips), and insides (possibly "entrails," i.e. a heart, arteries, blood, stomach, womb... and other organs) also. She was big enough for innumerable gods and monsters to live inside her.
Tiamat and (possibly) Apsu and Mummu from Sumerian mythology, especially the Enuma Elish, seem to fit the bill pretty well (the multi-headed dragon shtick is probably an invention of Gygax and Arneson, but the dragon bit, period, predates them by nearly a century, at least).
To give an idea of how monstrous Tiamat was, one interpretation of the Enuma Elish depicts a battle between her and Marduk, an invader of her domain. By the end of it, Marduk slays Tiamat by decapitating her, making her "upper half" (her skull) into the earth and her "lower half" (her tail) the sky.
Girtablilu, created by Tiamat, was so large that when it stood up, it cracked the sky.
Lotan of Canaanite folklore, a seven headed sea serpent who was either an aspect of a greater god called Yam, his pet and attack dog or part of the cosmic ocean. Lotan possibly becomes the seven seas after being defeated by Baal-Hadad.
Lots of people view Aztec Mythology as being the worship of a bunch of Eldritch Abominations due to copious (though likely exaggerated) Human Sacrifice. But the purpose of all that sacrifice was to keep the Aztec gods (especially the sun/war deity Huitzilopochtli) strong enough to defend our world against the depredations of the horrific Tzitzimime—skeletal, spider-like star demonesses with flint blades for tongues and rattlesnakes for penises. (And yes, they were still considered female.)
Even putting their bloodthirstiness aside, the Aztec pantheon is exceptionally strange and alien, even by mythological standards. Most of the gods had many, many different names, forms, appearances, roles, and general alter egos, to the point where it's often unclear whether two gods were considered separate entities or not. (Even archenemies Quetzalcoatl and Tezcatlipoca are not immune to this; Quetzalcoatl was sometimes called, "the White Tezcatlipoca.") They'd actually destroyed and remade the world four times before this one (which has to be some sort of record), going through four different suns in the process, and many of them took forms of the "hide under the bed" variety, often involving skeletons, Mix-and-Match Critters, things being substituted for body parts that really weren't meant to be, and lots of feathers and obsidian blades. Plus, their names all look totally unpronounceable if you're not familiar with them. After all that, copious requirements for Human Sacrifice almost seem like an afterthought.
But the biggest and baddest thing to come out of Aztec Mythology is undoubtedly Cipactli, the gargantuan monster that inhabited the primordial ocean that existed before physical creation. It's described as part crocodile (which is what its name means), part fish, and part toad, and as having an extra mouth at every joint. The gods couldn't get to creating with this... thing running around (one version of the story says that it ate all the previous versions of the world), so Quetzalcoatl and Tezcatlipoca actually put aside their eternal rivalry to kill it, and even then it managed to take off Tezcatlipoca's foot (to put this in perspective, Tezcatlipoca was the Top God and believed to be nigh-omnipotent) before finally going down. Then the two gods made the world out of its corpse, because it was just that big. The struggle of the gods against some primordial, horrifyingly incomprehensible thing before they can start doing god-things is actually a very common theme in world mythology; Typhon, Tiamat, Apep, Ymir, Amatsu-Mikaboshi, Behemoth and Leviathan, and even Satan's Great Red Dragon form from the Book of Revelation (all of which are described on this page) all have traits of this sort of story.
While most people (including neighboring cultures to the Aztecs, many of whom shared the same base mythology) would consider Tezcatlipoca to be evil and Quetzalcoatl to be good, like with any proper Eldritch Abomination, it's nothing like that simple. One of the jobs of Tezcatlipoca is to ensure that Quetzalcoatl does not return to full power... because if he did, the resulting cataclysm would destroy the world and kill every living thing. Again.
Silla of Inuit cosmology and most of its variations recognized by related cultures. Basically it is the air every thing breathes and the main ingredient of everything's soul. It is not exactly benevolent either as it wants those parts of itself that make up all that lives back, to the point it will convince kids to wander out into the tundra where they will freeze to death. It will not be completely satisfied until there is no more breathing.
Several Native cultures have tales of the wendigo. In some, it's just a human twisted into a monster by cannibalism. In others, it's a primal spirit with no fixed shape. Anyone whom it reveals its true form to can become host to it, and it drives people insane merely by existing. As long as it has any possible hosts left it cannot die, and it can shift from host to host instantly.
Other Mythologies and Religions
Nautical Folklore: The ocean. Be respectful to it and it will feed you and make you rich. Disrespect it and it will drown your homes. Or just makes sure that you will never be heard of again. Also home to more countless eldritch abominations.
His Archons are actually described as this, their real forms being mind-numbingly horrifying (and one the them is a solar deity). They also eat human souls and may be aspects of the Leontoeides/Ialdaboath (the snake lion thing mentioned previously).
The Olympic Spirits (no, not those ones, these). While the author was pretty much forced to depict them as minor deities (cos the Inquisition and all), their pagan roots are barely disguised, and they are hinted to be something other than what they are described. The sheer historical mystery about these entities is enough to remind one of Arthur Machen's works.
The Nameless Thing of Berkeley Square gives off these vibes. It's a monstrous, tentacled creature that allegedly took up residence in a building in Berkeley Square and killed several people. It's really telling that the only way they could name it was to call it a "nameless thing".
The Bunyip of Australian Aboriginal myth - there is no agreed-upon description, and it can be anything from a Small Annoying Creature to Ultimate Evil. According to Yowies and Bunyips and Drop Bears, Oh My, "Most accounts describe it as some sort of large carnivorous, aquatic creature that dwells in billabongs (seasonal lakes) and rivers, preying on unsuspecting travellers. Some variants claim that it can become invisible, or take the form of a beautiful woman to lure in victims."
Iku-Turso of the Finnish national epic Kalevala is described as a giant octopus capable of walking on land and blighting out the skies with a magic tree. Fortunately Turso was defeated by Väinämöinens magic music.
Technically, the appearance of Iku-Turso is never really described in Kalevala, but since it is very similar to the Tursas, the Finnish equivalent to the Kraken, it's usually descipted as looking like the Kraken, ie. a big octopus-creature.