The Muck Monsters of Mordo were aliens who attacked Spaceman Spiff while he was piloting his spaceship in the vicinity of Planet ZK-5. They fired a fatality-flare at Spiff which missed by mere micromips. They broke off from their pursuit when Spiff flew his ship into the rocky, icy rings of the planet.
- The Muck Monsters themselves were not seen, only their spaceship in the distance as they pursued Spiff.
This encounter was apparently a metaphor for Calvin playing dodgeball in gym class with Moe.