Angered by Trick or Treaters coming to his door, and learning that Halloween originated on Earth, Maximus sent mutant pumpkin seeds down there to attack. They landed in Moosejaw Heights, growing into mutated pumpkins, or Jacks. The creatures began consuming all the Halloween candy, and attacking them caused them to reform into more.
Betty managed to use X-5's new bottomless loot bag to ensnare them all and place them in Maximus' ship, after scaring Maximus off of Earth, he unknowingly took them with him, where they began attacking his citadel.
The monsters possess pumpkins for heads, with thorny tentacles as bodies, arms and legs.
The Pumpkin Monsters were single minded, intent on invading and consuming every piece of candy in sight.