A Zargon Slug is a giant, alien sluglike creature.
Zargon Slugs are carnivorous, and very ferocious. They are used in the Polyhedron Fortress to guard vaults. One was seen in "The Notorious Mrs. Goodspeed" when Gary Goodspeed, Clarence and Sheryl Goodspeed broke into the fortress. According to Clarence, the slug they encountered there was actually small for the species.
The slug could easily keep up with the cycle Gary, Sheryl and Clarence used to escape the fortress with, and shrugged off any gunfire fired at his mouth. Sheryl was able to the beast by aiming at it's back however.