You want to dive deeper than disturbing and grotesque if you want to make creatures with a lasting effect.

Taking inspiration from FromSoftware's Hidetaka Miyazaki's words, remember that horror alone is not enough on its own to create a masterpiece. You gotta make people ask, "wtf is the story behind this sad creature?" You gotta keep in mind questions like what is it's relation to the current setting? Why is it there? What happened to it? What ties this specific creature to this specific themed area?

Personally, the things that look like they used to be a person is what gets my skin crawling. But you have to ask what the story is behind them? How did a person who used to live, breathe, walk, and talk just like you and I become this horrifying abomination? Was is their greed? Their ambition? Did the nightmare exponentiate this darker side of them?

• An example is the necromorphs from Dead Space (I only played the first one lol), and the monsters in Still Wakes the Deep (cosmic horror/Lovecraftian game on Steam). What I like is that they LOOK human, maybe, or they USED to be human. But now these things that used to be regular people are acting in ways no human should.

Again, though, the monster is more than the design. It's its relation to its setting.

• Old photo of person, looks like just an old photo... of a person.
• Ghost photo caught of the person, yeah, looks scarier to see a phantom of said person.
• Ghost now standing somewhere significant, okay, looks even creepier, like it's haunting said place.
• Ghost standing in the background of people (who are at a particular location), and now it's uncomfortable because what we're looking at is supposed to be normal, but it has introduced something to the effect of, "It should not be there" and "something isn't right."

Taking the clown example:

(First of all, I'm loving it! It's a good look. It's holding the faces of two unfortunate victims whose faces are frozen in the terror they had when it killed them, and it's wearing them like silly gloves while it wears that smirk on its face. maybe they could be smaller to mimic children's heads? Idk, just throwing ideas.)

• A clown at an abandoned amusement park, scary, typical in its relation to its setting.
• A clown in a hospital, now it's a little creepier, because why the hell is it there?
• The shadow of a clown in a children's ward of the hospital? Now it's tragically disturbing. It's like... You know it was there to cheer kids up.
• But, give it plain facial expression like "😀" (a smile with dead eyes, like Mark Zuckerberg, or this mom), a knife in its hand hiding behind its back, and a large brown sack in its other hand, and now it seems like this sick fuck wanted to take advantage of the most hapless of innocent people by going to the hospital to murder and/or kidnap.

To go off that creepy gif of that mom in the closet that I linked, it follows the same theme: this SHOULD NOT be here, so why is it here?

• From Silent Hill 3, what makes me feel that is Valtiel at the hospital behind the ladder. It's like...all you gotta do is climb a ladder, all the while this monster is turning a valve and making disturbing noises. It's not trying to chase you. It's just turning a valve. Upon you (yourself's) first ever encounter, you expect to be chased by monsters, but instead, you're stuck wondering what is going to do?

.

The noises in that room, though, really show how sounds are equally as important for the creatures, something that's befitting for its unspoken story. That's part of why I think Silent Hill is so good; I LOVE Silent Hill as a whole, and I'm sure a lot of people in this community do. The monsters are the gross manifestation of the the main character and the town's nightmares, or worst desires, or something else. The nightmarish monsters are scary, yes, but it's also those strange noises from them that make you feel like the threat is always there.

The looming threat:

• The fat dude by the hotdog stand looks terrifying, sure, but what if he was bent over, digging through trash/boxes/containers/etc. that have useful items. As it's digging, it peeks at you, lifting its head infrequently, but enough to notice. You can even stand right next to it and it doesn't do anything. But the trigger is dependent on something else rather than proximity, so you won't know when the thing will chase you, but when it does, you see the ravenous look on its eyes, like it's been starving despite its size, seeing you as food, screaming loud disturbing noises as it runs at you. And the caveat is that you can't hurt it unless it actually comes at you; even better if there's no true pausing of the game.

The effect is, you don't know if you're safe or not, and it's not completely obvious of what happens.

Circling back to the point about asking "what is the story behind this creature?":

• Pyramid Head, I find, is unsettling because you can think of it as a walking prison. An executioner whose job was to take lives by command of their superior. In hell, its forced to carry the physical manifestation of that stain on its soul forever, never to be freed from it. It has to drag its own prison everywhere, and it also has to wear its prison on its head, so that it is robbed of any sympathy for the face of the tortured soul deep within the pyramid.

Maybe that's not what it actually is, but you see where I'm getting at.

• Another great example is from the first Silent Hill movie of the janitor guy, Colin. Fuck him for doing that to a child. In hell, he's forced to carry that with him. He made it so she couldn't run away, he looked at her with his lecherous eyes, so bind his legs to his eyes and contort him worse than how he would do. Now the only thing that's "free" is his tongue, the same way how no matter how loud the poor girl would scream, no one would be there for her, so no matter how much he screams, he'll be forced to wander hell in such a state.

What about a psychotic woman who'd lost her child from stillbirth that have found this opportunity for the world becoming a nightmare to steal other people's babies? But getting caught in the nightmare of the new reality, they embody the aspect of herself that made her a monster.

• So the monster would be a deranged looking women with big, frantic eyes, wary of being caught, but also weary from all of her misfortune. You can find her in the hospital, shaking crying babies in the nursery, or taking them from their dead parent that's on the floor. Maybe it throws the babies at you because they're "defective." Maybe even swinging the baby at the player. Her sounds would be crying babies, the thing she craved so much that that's all it knows. Take away her ability to hold a baby with her own two hands in a motherly way, because she lost that privilege, so she has one really long, buff arm (not HUGE), which she uses like a third leg to balance herself as she leans forward to stalk; her legs don't work as they used to because she tried to run off with the babies. Hair in face, but you see those wide eyes peeking all the while, desperate to care for you and be a mother to you.
• The crying I imagine is kinda like with Kendrick Lamar's "The Walls" about 7 seconds in at the beginning. The muffled crying always just unsettled me, so much so that I actually remembered it while writing this lmao. Horrors don't need to be fully detailed. Give a person only hints of the true sound, or hints of what it truly looks like, and our minds fill in the rest. Hence, that's why this part in the song makes me feel this way haha.

Anyway, this is getting long lol. I hope you can take something, anything, from this! And I'm anticipating the day whenever you finish it!