So how does this happen in your fictional setting? How do the monsters develop in such a way they can threaten others? Who tackles those threats? What is the impact of those monsters and how can others recover from that/limit the damage? How can the monsters hide what they really are as they develop their scheming? And what is it they are ultimately seeking to achieve?
As for the more obvious monsters, do they threaten others because they themselves are threatened and feel the need to fight for their right to exist? Shrek is a good example of this theme.
And what about those who despair at the damage done by monsters? How can they overcome that despair to reach out to help those who've been hit the worst by these creatures? What acts of sympathy/heroism here can develop your other characters and add depth to your story?