Unicron from Transformers: The Movie; 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 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's 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 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 comicbook) 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 as being 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":
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 neigh unstoppable is the pure virulence of The Thing. One cell. Thats all it takes 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.
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.
In the Finnish horror-comedy Rare ExportsSanta Claus is depicted as one. That creepy guy from the traileris just one of his elves. The real Santa is... creepier.
And seems more to be a variation on Krampus than Santa Claus...
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 'peace' where violating the smallest rule results in being robbed of your free will.
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.
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.
The evil room in 1408 is explicitly identified as never having been anything even remotely human.
Galactus gets turned into this in the Fantastic Four part 2.
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.
