The Sinister Slug is an enemy of Mermaid Man and Barnacle Boy. He first appears in the episode "Mermaid Man and Barnacle Boy II" as a cardboard cutout, and later in "The Bad Guy Club for Villains."
Sinister Slug in "Mermaid Man and Barnacle Boy II" is dark green, has red eyes, and has a purple symbol on his chest.
In "The Bad Guy Club for Villains" he looks far different.from his first appearance. He is a light beige color, full red eyes, sharp teeth, amd the symbol on his chest is now gone.