|First appearance||Into the Wild Green Yonder|
The Dark One was a desert muck leech that lived on Mars. He was almost killed by Leo Wong, but Leela saved him from poisonous cement that had killed the rest of his species. He again escaped death when Zoidberg almost ate him. Again, Leela saved him and he became the Eco-Feminist mascot. He was later zapped and killed; Zoidberg ate the remains.
- The Tunneling Horror from Bender's Game bears a striking resemblance to the Dark Ones. The Tunneling Horror was either an inspiration for the Dark One's appearance or the Tunneling Horror is what the Dark One's became in the fantasy world.
- Technically, its genus is a genus of killifish in real life.