This is not terribly surprising considering the mystery of their origins, but the commenter also provided proof.
This is a sealed box of the candies that the monsters were sold from. It translates to Halloween Coffin Surprise and you can see the Bat on the cover.
I was planning on delaying the next blog post until next week (in hopes of getting a weekly discussion pattern), I felt this needed to be addressed immediately. I am going to update the Cromy section immediately.
For now I plan to leave the section on this site, despite not being MIMP. The reasons are two-fold.
First, many people have collected these as MIMP and will probably look for them on a MIMP site. Secondly, there are few English sites that even mention these figures, so I think it is best to leave it here for now. I may eventually make a small site just for them.
Huge thanks to Gianluca Spinoso for the picture and information about the monsters.