Eldritch abominations are one of my favorite types of monsters. I like it when a monster is inherently “other” in both appearance and behavior. I’m talking about masses of tentacles and eyeballs appearing through a rift in space or a vaguely humanoid entity that on closer inspection has a giant mouth in place of a face. They’re a staple of cosmic horror but can appear in anything with a hint of sci-fi or fantasy.
What sets an eldritch abominations apart from more traditional monsters is the way they break the rules of reality. Lifeforms on Earth don’t have tentacles outside the ocean and eyeballs are usually in a specific spot on the head rather than placed haphazardly across the entire body. These features go against what is considered normal. The uncanny valley can also be a good determiner of an eldritch abomination. Monsters that can be mistaken for human aren’t a thing in real life.
This rule breaking also applies to abilities. A bear can easily kill a human, but it does so with its immense strength and sharp claws which is completely within the realms of what makes sense. However, a bear that kills people because making eye contact with it causes your heart to disintegrate would not be how reality works.
Admittedly, what separates an eldritch abomination and a “normal” supernatural being may be rather arbitrary. All supernatural or magical elements break the accepted reality, but not all do so in the same way. A vampire or werewolf are such well known monsters that their inclusion in a work is almost mundane due to how often they are encountered in media. In this way, familiarity is a major factor that separates eldritch abominations from other magical beings.
A series with good examples of eldritch abominations would be Otherside Picnic. The monsters in this series come from another dimension called the Otherside and take on the forms of monsters from creepypastas and urban legends. The threats include ribbon like creatures that drive you insane when you look at them, an unnaturally tall woman that lures people in by taking the form of a loved one, and mummified monkey corpses surrounded by hovering teeth. At one point the protagonist, Sorawo, is being chased by dogs with human faces and is terrified by the fact that she has no idea what will happen if the dogs catch her. The monsters in Otherside Picnic break the conventional rules for reality and remain distinctly otherworldly even as the protagonist’s understanding of them grows.
Overall, I’d say that eldritch abominations are a unique and interesting type of monster. Their otherworldly nature gives them an edge over more traditional types of monsters in terms of danger and horror. They come in many sanity shattering forms, but their most distinctive trait is the way they break the rules of reality. Eldritch abominations are an aberration that shouldn’t exist.