How about the other words here? "The" suggests this monster is the only or most consequential one in the area. "Has" is present tense, which brings the story into the moment. Look at the difference a single letter makes between these two versions: "The monster has been here." "The monster had been here." "Has" is closer and more urgent. Also, because most of the stories we see in the present tense are also told by "I" – that is, in first person – "has" suggests that the narrator will be telling this tale directly. "Been" is neutral. Like all forms of the verb to be, it links other words with minimal shading of its own. The monster did not stand or rest or shred or kill or act in any more distinct way here. It existed here – which is worrisome enough. Finally, "here" is another word that brings the action of the story close to us. In this location, where we are, there was recently a monster. The monster might not have gone very far.
A first sentence with tension or suspense is a hook. "The monster has been here" is definitely a hook. A first sentence can also be a promise – a sentence that creates an expectation about a story that the rest of the book needs to fulfill. With the word "monster" and the implied narrator, we have two characters. Monsters are usually villains and protagonists who tell their own stories are usually heroes. A villain plus a hero sets up a conflict. So this sentence suggests that the narrator and the monster will eventually face each other and fight. That is a promise.
The word "monster" also suggests the genre of the story. Monsters appear in fantasy and horror, and can only belong to realistic fiction as metaphors. This sentence is blunt and straight-forward. There's nothing to suggest that this is a figurative monster. While the monster could still turn out to be, for example, an abusive father, an author who wanted to use "monster" in that way would usually give a clue in voice or other details. Instead, Roanhorse has surrounded the fantastic detail with ordinary words, a strategy used by authors like Stephen King to add reality to supernatural situations.
I've read and enjoyed the entire book. The monster is literal and powerful.
Could any five words have this much impact? No. I make a habit of choosing the first sentences that interest me. Only a tiny percentage of books start with a sentence that hold my interest enough for me to write about them, and very few of those are this short. Rebecca Roanhorse has written an excellent hook and promise.