Fish Monsters are recurring hazards in BFDI. They are big, green, snake-like monsters with sharp teeth and multiple tongues that can efficiently trap victims. They live in the Goiky Canal and seem to be omnivorous.
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.
In "Hurtful!", multiple Fish Monsters are seen. It is shown that the larger fish will eat the smaller fish as a large grey fish eats a small green one, then Leafy.
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.