Eldritch Abomination/Film

Obviously, that didn't fit in the drugstore.

Did you think the sarlacc from Return of the Jedi was freaky on the outside? Be grateful you didn't see the entire thing...
Unicron from Transformers: The Movie is a god of apocalyptic destruction encased in a metal-planet.
Cast a Deadly Spell climaxes with a cultist trying to awaken a Lovecraftian beastie. Luckily, the needed Virgin Power turns out to have already been, um, depowered. Yes, yes, Incredibly Lame Pun and all that...
"Mr. Shadow" from The Fifth Element. Never really explained other than being pure death-bringing evil from deep space. He/it/whatever even returns every so often when the stars are right.
The tentacle creature from the 1981 film Possession.
The Monster from the Id in Forbidden Planet, a manifestation of Dr. Morbius' subconscious. While analyzing a footprint cast, the science officer comments that whatever made the print goes counter to all known evolutionary theory and would be a nightmare in any world.
Gozer the Traveler in Ghostbusters, an interdimensional being who seems to have no fixed form of its own. It's taken the form of a "large and moving torb", a "giant sloar", and... the Stay Puft Marshmellow Man. Zuul and Vinz Clortho, Gozer's demonic dog disciples, have magical powers, but aren't quite at the Eldritch Abomination level.
Several of the space monsters from the Godzilla film have the potential to be this, but King Ghidorah's background in particular confirms he's been destroying entire alien civilizations for a very long time.
The first Hellboy film featured the Ogdru Jahad, depicted as a group of ginormous be-tentacled crustaceans inhabiting The Void outside the universe, as well as a tentacled monstrosity (referred to as Behemoth in the supplemental materials, though it was obviously based on the Ogdru Hem from the comics) that burst out of Rasputin's body and grew from man-sized to warehouse-sized in minutes.
The true form of the Tall Man from the Phantasm movies is implied to be some kind of Cosmic Horror that is anchored in another dimension.
Although none directly appear in the Japanese version of The Ring, it's hinted that Sadako's mother had contact with such beings due to her psychic powers... and that one may have been Sadako's father, rather than the man who was believed to be. Well, Mr. Towel says he's from the ocean, in any case.
The Hallmark channel version of Alice in Wonderland featured "The Crow", which scared Tweedle-Dum and Tweedle-Dee.
Pirates of the Caribbean has Calypso, the goddess of the sea whose favored form seems to be a vast swarm of crabs. The Kraken would also count, from the way Gibbs describes it.
John Carpenter used different permutations of the trope in the movies he appropriately called his "Apocalypse trilogy":
- In Prince of Darkness, Satan is described as being such an organism. Is also attempting to bring forth its true father, the Anti-God.
- The Thing is certainly capable of shifting into one, what with having the ability to mix-and-match the genetics of every other creature it's ever come into contact with before. It's also possible that its "true form" and/or "original form" are as such.
  - What makes it truly terrifying and nigh unstoppable is the pure virulence of The Thing. One cell is all it needs to survive. Given a host, within hours it can recreate itself to a substantial threat. The best solution to stop it is to nuke it head on. Even then, who knows whether this would truly stop it or just spread it around.

Blair's input into computer: If intruder organism reaches civilized areas...
Computer's output: Entire world population infected 27,000 hours from first contact.

There are 2 theories about Blair's rampage. Either he's been assimilated and the Thing wants to kick some ass, or he went mad from studying the nature of the Thing.

The unreal horrors trying to find purchase into our reality through the works of Sutter Kane from In the Mouth of Madness are definitely abominations. The numerous Shout-Outs to H.P. Lovecraft and the Cthulhu Mythos help.

Kagutaba from Noroi the Curse. A chaotic entity once used as a tool by a village of shamans, it stopped cooperating at some point, forcing the villagers to perform an annual ritual to bind it underground. It has no definite shape (although its influence is felt everywhere), but when it manifests itself physically... well, let's just say it makes for one of the most jarringly scary endings of any film ever.
This trope seems to perfectly describe whatever the eponymous ship from Event Horizon encountered after activating its experimental faster-than-light drive for the first time. Although mainly hinted as being a place rather than a creature, the ship is at one point said to have become some kind of malevolent entity. Quotes such as 'Hell is only a word. The reality is much, much worse.' suggest this trope. At one point, a character gets first-hand experience of what is on 'the other side' and returns catatonic and suicidal, only managing to whimper rather ominously 'I saw things...' Allegedly, a lot of scenes were regarded as just too horrific and were sadly cut. They can be found here for your viewing "pleasure".
In the Finnish horror-comedy Rare Exports, Santa Claus is depicted as one. That creepy guy from the trailer is just one of his elves. The real Santa is... creepier. Because he's based on the old Finnish tradition, Nuuttipukki. Nuuttipukki took Christmas away, scared children, and performed at people's houses in exchange for booze.
The Kamen Rider Blade film Missing Ace gives us Jashin 14, an ancient creature which has existed since the beginning of time whose power is reserved for the winner of the Battle Fight. It's a gigantic creature with four arms and incredible power. That's not even mentioning its role in Kamen Rider Decade, where it takes over the world and forces everyone into a 'peace' where violating the smallest rule results in being robbed of your free will.
In the film adaptation of Chronicles of Narnia: Voyage of the Dawn Treader, the malevolent green mist takes the form of Edmund's deepest fears in the form of a surprisingly Cthulhu-esque sea serpent. Earlier in the film, it's revealed that Edmund and Lucy read a lot of fantasy books, so this could potentially be interpreted as a suggestion that Edmund has read some of HP Lovecraft's work.
Dagon (2001).^[context?]
The D'Ampton Worm from Lair of the White Worm. It seems to be killed easily for an E.A., but it always gets better. It also has a race of vampire Snake People feeding virgins to it, so it stays well fed and hangs out underground a lot.
Raggedy Ann and Andy A Musical Adventure has The Greedy, who is not a goofy candy clown as the movie poster implies; in fact, he's a sentient, constantly shape-shfting mass of taffy that spews the candies he constantly eats. He then sings a song about how it makes him miserable because he desires a sweet heart - a literal one, not a partner as the song implies - which leads to him attempting to kill the cast in order to get one from Ann because she mistakenly mentioned she has one. The animation for the scene is pure gold and its idea is far ahead of its time.
Whatever the heck it was that was impersonating Freddy Krueger during Wes Craven's New Nightmare. Its description in the film is really vague, except that it's ancient, enjoys hurting people, and can only be contained when a story captures its essence (which Freddy, being the Complete Monster that he is, seemed to do). Minus the last part, it sounds a lot like Nyarlathotep.
Yog Kothag in Forever Evil.
The evil room in Fourteen Oh Eight is explicitly identified as never having been anything even remotely human.
Galactus is an example of this in Fantastic Four: Rise of the Silver Surfer, where he is more of an Energy Being loosely similar to his incarnation in the Ultimate Marvel comics, though he briefly resembles his 616 counterpart in a few shots.
The Mist is the story of a small town being opened up to a dimension full of monsters. The characters mostly fight off the smaller insect-like ones, but they do have a run-in with the tentacles of a monster that we never see the rest of, and the creatures in the page image are seen near the end.
Leviathan, the Bigger Bad of the Hellraiser series. It is a giant, floating black prism that feeds on souls and is constantly shooting an Agony Beam.
Death from the Final Destination franchise is an inhuman, primal force of nature [[Wild Mass Guessing|who may or may not be speaking through a character played by Tony Todd to spill its secrets to its future victims. It does this because you can never truly defeat it, and it knows this: It's omniscient and omnipresent, always lurking in the shadows with full knowledge of how everyone on Earth will die, and if anyone avoids the fate it had in store for them, all they accomplish is pissing it off and ensuring that their new death will be even more horrific than the one it had initially planned. And when it moves in for the kill, the laws of reality get thrown out the window as structures and vehicles suddenly fall into ruin, animals behave strangely, and seemingly safe rooms and areas become overly complex Rube Goldberg deathtraps.

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