Mary Shelley, in her novel Frankenstein, tends to fall away from the typical characterization seen in many horror stories. While Victor is described completely, Shelley's creature is contrastingly obscured from the reader.
In chapter five, when the creature first comes to life, the reader is given very little detail when it comes to the physical nature of the being.
I saw the dull yellow eye of the creature open; it breathed hard, and a convulsive motion agitated its limbs.
His limbs were in proportion, and I had selected his features as beautiful. Beautiful!—Great God! His yellow skin scarcely covered the work of muscles and arteries beneath; his hair was of a lustrous black, and flowing; his teeth of a pearly whiteness; but these luxuriances only formed a more horrid contrast with his watery eyes, that seemed almost of the same colour as the dun white sockets in which they were set, his shrivelled complexion and straight black lips.
Previously, in chapter four, the description of the creature is even more limited: "about eight feet in height, and proportionably large."
Over the course of the remainder of the text, whenever Victor comes in contact with the creature, he sees the creature in shadow, as a blur, or fails to describe the creature at all.
The horror of this comes from the fact that readers do not know what the creature looks like, aside from being very tall with yellow eyes and skin. His appearance is shadowed from the listener in the same way it is shadowed for Victor.
What this does is it allows the reader to create a mental image of the creature for themselves. Every reader, essentially, could come up with a very different picture of what the creature looks like to them. The horror in this is compounded given the reader is doing exactly what Shelley wanted (as defined by her introduction to her tale): "O! if I could only contrive one which would frighten my reader as I myself had been frightened that night!" Shelley, then, unlike many horror writers who provide detailed descriptions of their monsters, does not wish to define the creature completely for the reader. Instead, she only offers the reader small tidbits of information about the creature in hopes that her tale will frighten her readers.