The Giant Fish Monster is a monster that resembles a huge one-eyed reptile/fish creature. He has a long tongue that can grab people.
He appeared in the Teenager Flashback in City of Clipsville, where he rose up peering at a dome. 2 passer bys screamed in fear as the monster grasped one of them with his tongue and ate her. Bubbles didn't notice him when she was talking with Boomer.
He feels no pain when he lashes his tongue out of the dome.