Wow, what a wonderfully unnerving short story!
It's told from the point of view of an abused child, who is kept chained in the basement by its parents. It is written with odd grammar, bad spelling, and strange phrasings to reflect a complete unfamiliarity with normal human speech, writing, or the world outside the basement. The child is curious about the laughter it hears from Upstairs--and the glimpses it gets through a window of children playing outside-- and is punished by beating when it goes to look. A few violent things happen towards the end, including a promise of even greater violence to come.
This is all pretty terrible, but what makes the story truly creepy are the vague hints that this child is literally not human-- and that may be some sort of giant insect, alien, or a monster. It describes itself as bleeding green, for instance. And it talks about crawling on the walls with all its legs. That said, it's still not fully clear whether this is true. Given the narrator's ignorance of so many basic things (it doesn't recognize a cat for instance), does it actually know the difference between green and red? Or walls from the floor-- or legs from arms? The reader is left to wonder. At the same time, the possibility of the child being truly, literally a monster leaves the reader to wonder whether the parents' actions-- though certainly cruel-- may somehow be at least understandable, which is possibly even creepier.
Overall, it vaguely evokes hints of Frankenstein and John Gardner's "Grendel" (which retells Beowulf from the monster's point of view). And all this in just 3-4 pages. Brilliant, IMHO.