After you read Frankenstein, think about what things classify this book as a horror story. In fact, what classifies any scary story as a horror story? What characteristics do you see in Frankenstein that almost all horror stories possess? Write a 5-7 sentence paragraph explaining your thoughts on this topic. You may choose to begin your paragraph with, "In Frankenstein, as with almost all horror stories, there are several defining characteristics of the genre. To begin with..."

I can think of a few ways in which Frankenstein counts as a horror story, though two may be a bit cerebral. 
The "monster" itself—The creature has been built from the parts of corpses, literally stitched together. It is larger than most humans and horrible to look at. This fits with the themes you see in the horror genre because it is presenting us with something repulsive that is meant to disgust us—otherwise known as the "gross out."
The obsession—Dr. Frankenstein's obsession with his studies and research turn him into someone that cares less about the consequences of his actions than with meeting his obsessive goal. He is consumed with what he is doing. We see this "mad scientist" figure in all sorts of horror tales, from Andre Delambre in The Fly to the character of James Hollister in Firestarter.
Self-inflicted doom—Dr. Frankenstein works very hard to bring his dead conglomeration to life, but when he succeeds, he does not love or nurture it; instead, he rejects it and sends it out to fend for itself. The creature then decides to exact its revenge on its creator by making him suffer. We see this in lots of different tales of horror, from Freddy Kruger (burned to death by the parents of his victims, then coming back stronger as a kind of demon) to Jason Voorhees (who, in some incarnations, comes back for revenge on the camp counselors who let him drown). In the horror genre, the trauma inflicted on the characters is often a result of their own choices. 
The eternal beast—In many horror stories, the "creature" or "horrible element" outlives the main characters with the threat of coming back.
Those are just a few thoughts, though I am sure there are others you could use.