Fish Monster is a recurring hazard in BFDI. It is a big, green, snake-like monster with sharp teeth and multiple tongues that can efficiently trap victims. It is known to live in the Goiky Canal.
It first appeared in Take the Plunge: Part 2. While Blocky was blocking his boat's leaking, a fish bites Blocky, causing him to shoot up in the air in pain. His team members watched in shock as the monster "eats" Blocky when he was falling down. However, in the next episode, it was revealed that the monster missed him and he fell behind the monster, creating an illusion that he was eaten. While Pencil was trying to get the supposed Blocky out of the Fish Monster, it wraps its tongue around her, but didn't eat her, as it was distracted by Flower flying in the sky.
The Fish Monster reappeared in The Long-lost Yoyle City, bigger than last time, when Pencil was once again wrapped in its tongue. Her team was forced to saw her in half to get her out. Later, Pencil kicked Rocky into the monster's jaws as revenge for barfing on her. In The Desert, Coiny attempts to hit the FreeSmart Van with a branch, however it missed and hit Rocky, causing him to fall into the monster. At the near end of the episode, Puffball was dropping everyone on her team off of her, which was devoured by the Fish Monster.
- In Don't Pierce My Flesh and Hurtful!, a creature similar to the Fish Monster, the Fire Monster, eats contestants that fall off the pole.
- It is possible for the Fish Monster to be a species rather than a singular monster, though no more than one as been shown on screen.
- It has a habit of latching its tongue onto Pencil, who usually reacts with "Yikes!".