Mary Shelley's Frankenstein not only exists as an example of a Romantic novel, the text also contains Gothic characteristics. The elements of terror and horror are necessary in any Gothic text. That said, the inclusion of horror and terror is necessary to qualify Frankenstein as a Gothic text.
The true horror of the novel does not begin until chapter five. It is in this chapter that Victor first comes face to face with his creation. Deemed an abomination by Victor, the Creature is hideous to look at. Its yellow eye, tauntly pulled skin, and exposed, through translucent skin, muscles and arteries prove to illuminate the Creature as a horror of nature.
As readers transverse the novel, Victor is stalked by the Creature. While at times Victor actually sees the Creature illuminated by lightening, there are other times where Victor only thinks he sees the Creature. The following two quotes illustrate both concepts, the actual visualization of the Creature and Victor's thinking he sees the Creature.
I beheld the wretch—the miserable monster whom I had created. He held up the curtain of the bed; and his eyes, if eyes they may be called, were fixed on me. His jaws opened, and he muttered some inarticulate sounds, while a grin wrinkled his cheeks. (Chapter Five)
I thought I saw the dreaded spectre glide into the room. (Chapter Five)
As stated before, Victor is essentially stalked by the Creature. After hearing the Creature's story, and agreeing to make the Creature a mate, Victor is left with a promise, or threat, made by the Creature: "Depart to your home, and commence your labours: I shall watch their progress with unutterable anxiety; and fear not but that when you are ready I shall appear.”
It is through this promise that readers are left with their own feelings of anxiety, given they know the Creature could appear at any time. Knowing that the Creature is capable of murder, readers have no clue as to when the Creature will appear and continue upon his murderous rampage. This alone speaks to the horror and terror found in the novel.
Outside of this, some readers may find it horrific that Victor brought back to life a being in the first place. Not only has he been successful at reanimating life, he has done so using a patchwork of different body parts. Played upon by media since it origination, readers have been "programed" to fear Victor's creation. Unfortunately for many, the miscommunication between the novel and the media has proven to be damaging. Many who have never read Frankenstein believe Frankenstein to be the Creature and not the creator. That said, some may find the change of identification to be horrific (especially those who appreciate the novel).