That's awesome!
Yeah, so you should write it up as a threat, but don't go nuts with it as a threat to the PCs in play, exactly right. What would it be, a perversion of birth maybe?
One way to think about a threat who happens to be a PC's ally is to consider that the threat thinks the PC is on its side, or part of it, right? So if this giant bug monster is a perversion of birth, it craves overthrow and chaos, so that it and its friend the savvyhead can thrive. Its moves, like attacking someone from behind, stealing something, ruining something - it places these at the PC's disposal. Or I should say, it does these when it feels that they're in the PC's interests, whether the PC wants it or not.
So, in play, just occasionally look at the world from this critter's point of view. What does it consider to be threatening to itself and its friend the savvyhead? What does it think should be ruined, for the savvyhead's benefit? What could it overthrow so that the savvyhead would get more to eat or get more sex or whatever it thinks the savvyhead wants more of? What can it do about it?
-Vincent