The Slug was a ghost named Sl'g who earned his nickname due to his slug-like appearance and name.
Slug was an inventor in the 26th century who had four assistants named Turek, Cowan, Gard, and Bethany. One day, the four stole Sl'g's secrets and planned to create a weapon of mass destruction. They needed the power source of the Nautilus, ship of the fabled Captain Nemo, so they went back to a time where the Ghostbusters captured the captain's ghost and stole the trap containing him. In order to keep the Ghostbusters from following them, they sent them to four alternate realities. The leader used a device to contact Nemo from inside the trap so he could tell them where the Nautilus was. After going back in time again, they obtained the Nautilus' power source and completed their weapon.
Sl'g rescued the Ghostbusters and told them that he was an alien and explained everything. Together they freed Captain Nemo and defeated the four time-traveling criminals. The four demanded that the Ghostbusters bust the Slug and Egon responded by saying he wasn't a ghost. Sl'g then revealed that he actually was a ghost and that he told them he was an alien so that they would trust him. Sl'g and Nemo then rode off in the Nautilus.
- On Cover B of Ghostbusters Issue #11, Slug makes a non-canon cameo in the E.