Monsters. The darling of many creepypasta enthusiasts. Most readers love them and a lot of writers turn to them to deliver horror. Whole stories are based around them. In a lot of cases, they are the focal point of the pasta, like the badass hero in an action movie. They are the ones who carry the 'scare-factor' of the story. And that is, usually, pretty bad.
A monster should never be in the spotlight of a story. In a lot of cases, the story builds up suspense and tension, only to reach its 'high' with the reveal of the boogie-man. All the 'umph' of the story rests upon the ghastly description of the bone-chilling monster. But even if the description is indeed terrifying, it will take something special to 'stick' with the reader, unnerving/scaring him for more than a few minutes.
That is because in stories in general, it isn't the appearance of the characters that matters. It is what they do and how they act, it is the events that happen around them, and the role they play in those events. The visuals the writer is painting for the reader are enhancing the experience, they don't build it from the ground up. In order for a visual to be effective, other factors need to be in place.
The same 'rule' applies in horror stories. Throwing a monster description on its own will accomplish nothing, no matter the build up. The monster needs not only to look menacing (or anything), but feel menacing too. That can be accomplished in many, many ways. From giving some backstory to seeing the monster in action (although the second option is a bit of a hit or miss).
To give an example, let's take a look at Looks Like We Got a Live One Here, Boys by Humboldt, one of the better monster-pastas on the wiki (in my eyes). Throughout the story, we see glimpses of the monster's powers/actions. That sets up the reveal of the monster superbly, which comes near the end and is accompanied by (spoilers) a showcase of the monster's desctructive instincts. That is not all though. After that, we are hit by a twist which does well to creep us out one last time before the story ends. The story is a 'traditional' monster-centered tale, but it does enough to spice things up, keeping things interesting, fresh and above all, disturbing and creepy. Humboldt (the writer of the story) didn't rely solely on the monster to deliver the plot and it shows.
And that is how most monster-pastas should be written. The monster shouldn't be the focus of the story. It should be there to enhance the atmosphere of a tale. The reveal of the monster, therefore, should be a tool to add to the plot of the pasta, not try to be its punchline.
This is getting a bit too long (and I'm too lazy to continue), so I better end this here.
PS: I realize I may have overused the word 'monster' in here. It is meant as a place holder name for 'the bad guy'. Essentially a monster can be anything from a serial killer to a spider with goblin heads as eyes.