I found a couple of interesting websites on the topic of monsters in literature.
Listverse has ranked the top ten “most horrifying of all monsters from literature through the ages”: http://listverse.com/2011/07/05/top-10-horrifying-monsters-in-literature/.
A. V. Club ranks 24 literary monsters based on the work of a costume company. “MorphCostumes decided to rank the scariest monsters in literature in a brand new infographic. To come up with the final ‘scream score,’ the company looked at a combination of appearance, power, and evil intent”: http://www.avclub.com/article/scariest-monsters-literature-ranked-226386.
But the most interesting article I found is entitled “31 Fairly Obscure Literary Monsters”: https://electricliterature.com/31-fairly-obscure-literary-monsters-163c993a3d12#.wwk08efgi.