Some Monster keywords don't have stacking rules like
Alien and
Robot from
Munchkin Axe Cop.
Some monster types don't have clear-cut keywords per se, but do have gameplay effects like Bearded Monsters from
Munchkin Booty or Gear Monsters from
Munchkin Steampunk.
And the new
Munchkin Steampunk: Girl Genius on Kickstarter seems to have a new Monster Keyword:
Geister. It is unclear if these will have a stacking rule. EDIT:
They, in fact, have the stacking rule.