I made up this term in this context after PolyGuy began to express what sounded to my new-to-poly ears something like fears of the "boogie man."
Seeing me as someone that had only a mono past, he expressed concern that I might be seduced away from an openness to be in relationship with him by some unspecified mono person that I might meet and date in the future. He apparently had some experience with this happening.
The Monogamonster, then, is the spectre of monogamy that threatens a poly relationship, but may refer to monogamous partners who may raise that spectre should a poly person become attached to them.
Interestingly, I actually did have something of a Monogamonster run-in right about the time I saw the new Where the Wild Things Are movie, so now, in my mind, the Monogamonster looks like Carol, the jealous possessive character in that movie. :P