For such a campy, silly show about a Manchild who reads comic books, this thing is terrifying. For one thing, it seems to lack physical form. And from there, it all goes downhill. It's so deadly, the guy fighting it became a Multiversal Conqueror feared throughout creation just to stop it.
The "Machinations of Worms" arc centers around one of the Entity's "cousins", the King of Worms. According to the Absent Grimoire, it can feel fear, which warped its face to the point that anyone who sees it can recognize what it is. As such, it refuses to show its face, preferring to work behind the scenes through its "clockwork soldiers". Its ultimate goal is to master fear and heal its face. Oh, and it's also a lot smarter than the Entity, actively sabotaging Linkara: so far it took control of Linkara's cybermats, used them to infect Harvey Finevoice, Linksano, Ninja-Style Dancer, and the Zeonizer, and directly took control of Jaeris's A.I. Sierra.
The CreepypastaDogscape has the world entirely turned into Meat Moss made of dog parts except for a few humans, thanks to an entity called "the Dog Mother." A soldier manages to claw his way down to it in a fit of Unstoppable Rage, but it quickly kills and assimilates him.
The "Jackal" in a Creepypasta called "The Cave". Carvings of it found in the cave are all mostly scratched out. When the protagonist sees it, it's so incomprehensible that it really "looks" rather like that.
Lovecraft was right, he just got the details wrong. The Great Old Ones that invaded this reality at the beginning of time were imprisoned by the Powers That Be (or at least the Powers That Were at the Time), and in the modern day, the heroes have to deal with their remnants, their cultists, and their attempts to re-invade reality.
The Cthonians, the mortal descendants of the Great Old Ones, want to free their ancestors from their mystic prisons. The heroes naturally think this is a bad idea.
One, Thackerzod (Sweetie Bell possessed), is even part of the main cast. Lovecraft was apparently a big inspiration for FiMFlamFilosopher and the team.
From Rainbow Dash Presents: Haunting Nightmare, we have Smarty Pants, who possesses Twilight Sparkle and plans to be eaten by an even larger eldritch abomination so it can continue it's unholy life cycle. It only fails to do so because Thrackerzod has already laid claim to Twilight itself.
Another Rainbow Dash Presents, A Star in Yellow, has Yellow Star trying to summon one using Twilight as part of a Funny Background Event. Less funny, Yellow Star and her twin sister were tricked into summoning one, and were apparently fathered by one. On a Pony Abomination level, in light of this episode the parallels between Yellow Star and Celestia seem to imply that Celestia is actually a lot better than she could have been.
The Transapients, and especially the Archailects of . The very least of them is as far beyond a baseline human as that human is beyond a non-provolved animal. They are so advanced that their individual thoughts are sentient. Their physical forms vary from "Moon Brains" to entire pocket universes with just a small piece sticking out into reality. And most of those listed are "friendly" to humanity, though there are also entities that turn entire colonies into masses of flesh and metal as "art".
And then there's the Leviathan, a mysterious and really freaking hugenote approximate mass: 100 billion suns, approximate size: 10 light-years across object that has made contact with the Triangulum Galaxy, and is probably heading to the Milky Way next. Who built it? Unknown. What are its intentions? Unknown. How was it built? Unknown. What even is it? Unknown. Even the Archailects are confused by this thing's existence.
The Cjopaze from Ruby Quest is apparently the force responsible for downfall and corruption of the facility and also the reason why the Brig/reality has been tearing apart over the course of the story.
In There Will Be Brawl, we get the multiple Mr. Game And Watches once they use their Final Smash. This is especially surprising because they are seen all over the city earlier, socializing and holding down normal jobs.
Thrilling Intent has the Tax Goat, a creature that "repossesses" people and takes them to a different dimension. The series also features a shadow monster in the Prison of Lights that is notably the only monster the group never even tries to fight.
The Big Bad of browser game Demon Thesis is one of these, and is responsible for both empowering four college students with Elemental Powers and other spells and creating the monsters they are suddenly forced to defend themselves and others against, all for his own sinister purposes.
"Lets face it. You were made to say fuck it. Thats not an answer that lasts."
A series of 5 images done by a William Burke depict Garfield as some sort of abomination which switches between appearances for each image. In response to these images, Youtuber Lumpy Touch took the images and made 5 short animations called "Garfield Gameboy'd"
The first image depicts a large and rather bloated Garfield with elongated arms holding Jon with a single hand and asking for his lasagna.
The second image shows Jon hiding behind a wall with a giant Garfield head on the body of a spider. Jon looks very disturbed, and Garfield once again asks for his lasagna, emphasizing the word.
The third image shows Garfield as barely having any form anymore, being a huge pile of flesh with pincers coming off. Jon is hiding behind the television looking terrified, and Garfield simply says "Jon I can smell you"
The fourth image shows Jon hiding behind a door. Holding a shotgun, he appears to have shot Garfield in the eye, it oozing out of his head. Garfield, who now has the body of a centipede, simply smiles and says "bullets don't work Jon".
The final image depicts Garfield as a gigantic entity now towering over Jon, with a bloated body and thin legs. His eye is still blown out from the shotgun, but he slithers a large tongue towards the tiny Jon before him and says "The world is going to end Jon".