Your first decision is whether you want to write an “essay” or “an informative paper.”  In an essay, you defend a controversial point of view (“Grendel was not a monster”) by logic, quotations, etc.  An “informative paper” sorts out a topic and presents an overview of information culled from disparate sources (“War stories always tend to include nonhuman behavior.  For example…”).  In this kind of paper, you are free to bring in any examples that strike you as germane.  You are not required to “convince” the reader, but merely make some connections in his/her mind that are new views for him/her.  Your best procedure, then, after choosing one or the other kind of paper, is to make a list of the literature you want to deal with.  May I suggest something like “Monsters are a literary device many authors use to describe the unacceptable social behavior of some struggles a hero faces.”  Frankenstein, Grendel, robots (Asimov), natural forces in Greek legends (Scylla and Charybdis?), the alligator in Peter Pan, etc.