It could be about a strange, unknown creature lurking in the shadows. Maybe it's a creature that only reveals itself at night, and the two sentences describe the moment when someone first senses its presence and then realizes the danger it poses.
The two - sentence horror story about the creature could center around a creature that haunts a particular place, like an old abandoned house. The first sentence could introduce the eerie feeling one gets when approaching the house, and the second sentence could be about the sudden appearance of the creature and the sheer terror that follows. It could be that the creature is some sort of malevolent spirit or a mutated beast, and the two sentences are crafted to quickly build up the horror in the reader's mind.
The creature might be of a malevolent nature. It could be a demon - like entity that thrives on fear and chaos.
Perhaps it's related to a cursed object. Say, 'I heard it too. The old locket that moans when held, foretelling doom.' This implies that the locket has some sort of malevolent power and the sound it makes is a sign of bad things to come. It could be that whoever holds the locket is doomed to a series of misfortunes, like strange accidents or the appearance of apparitions in their lives.
Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.
I heard strange noises coming from the attic. When I opened it, a pair of yellow eyes stared back at me.
I found an old diary in the attic. The last entry said 'They're coming for me, and now they know I'm here' and then the pages started to bleed.
The little girl found a doll in the forest. As she picked it up, the doll's eyes started to bleed.
The bag man lurked in the alley. I heard his raspy breath, then he opened his bag and darkness swallowed me.
Perhaps the creature has a pouch or a hidden part of its body that is opened and closed by a zipper - like mechanism. And every time it opens or closes this part, it makes that noise, which could be a sign of it getting ready to attack or devour something. So the zipper noise is a warning of its malicious intent.
A bag man stood outside my window at night. I blinked, and when I opened my eyes again, he was inside, his bag oozing something foul.
Isolation is another factor. Consider 'I was stranded in the old house. I felt eyes on me from every corner.' Being alone in a spooky place and feeling watched, even though there's no visible source, is a classic horror trope that makes the story very creepy. It makes the reader feel the character's vulnerability and fear.