What makes them monsters in the eyes of the majority of your fictional world (and are they right to think this? The majority doesn't always get things right). Where do the monsters live? What do they prey upon? How are the monsters overcome (so they don't run riot and destroy everyone)?
Can there be friendly monsters (see Roald Dahl's BFG)? Can these monsters find acceptance or is everyone biased against them because of what others of their species do? Do the monsters recognize themselves as monsters? What are their intelligence levels and can they improve themselves?