Gothic literature contains different elements which are meant to bring about terror or horror in the reader. The characteristics of Gothic literature focus upon settings which are in states of decay or ruin, characters who struggle with mental illness or irrationality, and images of death, supernatural beings, and the things which instill fear in the reader.
Sometimes people fear the irrational. In a sense, beings such as witches, vampires, monsters, and ghosts all represent the irrational (given they, for some, cannot exist). The thought that these phantoms can walk the earth instills fear many. The inclusion of beings such as these tend to match the setting of the texts (dark, haunted, and frightening) and the actions of the texts (science experiments gone wrong (As with Mary Shelley's Frankenstein), the dead unburied (E.A. Poe's "The Fall of the House of Usher"), and sailors haunted by dead birds (Samuel Taylor Coleridge's "The Rime of the Ancient Mariner")).