Character
The Sinister Slug first appeared in the episode "Mermaid Man and Barnacle Boy II". He also made a second appearance in "The Bad Guy Club for Villains". He is a slug that most likely lives in sewers. His appearance in a game is "The Yellow Avenger", where you have to confront him in Jellyfish Fields.
Appearances
In Mermaid Man and Barnacle Boy II, he has red eyes, and resembles a green worm. He also has very sharp teeth.
In The Bad Guy Club for Villains, he has a much different appearance, with a bulbous beige body, completely red eyes, a few tufts and even sharper teeth. He also doesn't wear any kind of clothes this time.
Sinister Slug when he was younger as seen in The Bad Guy Club for Villains