Scholars believe these weirdos herald the end of the multiverse by entropy.

Personally, I agree with Bogleech these guys should not be a natural part of the rust monster life cycle, but I do like the idea of them having a connection.

My favorite idea is they are descended from dragons who were cursed into the form of rust monsters, and managed to partially beat the curse, creating a new variety of dragon, with a larval stage almost indistinguishable from rust monsters.

 also make the wings more like shrimp fins and less butterfly 

Personally I would put them on the material plane and make them sort of unwanted self-help guides with ulterior motives: they want to free you of your material possessions… by eating them.