The Dark One (born May 18, 3004 - June 30, 3009) was a desert muck leech that lived on Mars and also served as the main antagonist of the fourth film, "Into the Wild Green Yonder". 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 Dr. Zoidberg almost ate him. Again, Leela saved him and he became the Eco-Feminist mascot. He was later zapped by the Last Encyclopod 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 became in the fantasy world of Cornwood.
- Technically, its genus is that of the killifish in real life.