Let’s compare two scenes. I will not use any additional language trickery to make one better than the other. I don’t want to taint the results. The goal’s simply to show which is more frightening, so you can better construct a horror scene.
A man is eating breakfast when he hears a loud sound. He goes outside and sees a giant cyclops walking through his neighborhood. It turns and glares at him, baring its fangs.
A man is eating breakfast when he hears a loud sound. He goes to his window. A giant foot lands outside his window. The man backs away. He sees something move to look in the window.
Now, I must ask: which is scarier? Probably the second one, yes? Well, this is a more direct example of that old rule “show the monster as little as possible.” The rule exists not because a big monster can’t be scary, but because denying information forces the mind to fill in gaps. And, as that other cliché goes: “the human mind can scare itself better than anything else can.”
In the second, all we see is a foot. Could be anything’s foot. We know what a cyclops’ face might look like. We’ve no clue what this other monster looks like—so it looks like whatever the reader will find scariest.
The only danger is you can’t play this game forever. Horror audiences want to see the monster. The gruesome sight is the payoff. It’s the resolution. But, like a romance writer might delay a couple getting together—and thus makes the reader more invested in it happening—so too must we do this with our horror scares. In certain subgenres, it happens sooner, but you can’t go wrong with teasing it out for a little while. It’s scarier that way.