Life on Earth
Cetus was created by the sea god Poseidon to punish the region of Ethiopia for the the insolence of its queen Cassiopia, who boasted that her daughter Andromeda surpassed the Nereid nymphs in beauty. As the sea monster destroyed the shores of the region and terrorized its people, Cassiopia and her husband Cepheus were advised by their court oracle to sacrifice their daughter to it in order to appease Poseidon. The royal couple agreed by chaining Andromeda to a rock next to the sea so Cetus could prepare to attack and devour her. Fortunately for the princess, the hero Perseus stumbled upon the trouble while flying back to the island of Serifos with the head of Medusa. Instantly falling in love with her, Perseus swooped down on Cetus' back
Cetus was first seen in the beginning of the film with Scorpio, Leo, Lacerta and the Roc as Eris was demonstrating how she will make chaos to the Earth, planning it by stealing the legendary Book of Peace as she spotted it being transported to Syracuse. She sends Cetus to attack the ships. While Cetus attacks Sinbad and his crew, Spike went after the sea monster and bites one of his tentacles. Cetus flings Spike into the air with a ripped tentacle in his mouth before being caught by a crewman. Cetus shoots his long spiked tongue and swallows one of the crew members. Sinbad sets a cannon and Cetus grabs it and it exploded inside him and he threw up the crew member with green slime everyplace. Cetus prepares to finish Sinbad by eating him, but he stabs his tongue with a sword and two sharp sticks on two masts were sliced off and they landed on Cetus, deliberately killing him. Cetus sinks into the ocean and grabs Sinbad.
Eris appears and puts Sinbad in a large air bubble. She explained her sea monster is deceased because of Sinbad. Sinbad apologized for injuring her sea monster due to the creature's attack. After they were finished talking about Sinbad himself getting the Book of Peace for Eris, she released him and Cetus appears again behind Eris and he shown fine, well, and alive. In another scene, Cetus was seen with the other monsters, while Eris was taking a bath and she wanted to see more action. She turns the ocean into ice and summons her Roc.
When Sinbad and his crew made into Tartarus, he and Marina both get inside. While falling, all of the remaining monsters, including Cetus, watch them from above as they continue falling.
- Although Eris said Cetus was dead, it shows Cetus was alive and well for a stange reason. Possibly since he is a constellation. He's capable of turning back into a constellation at will after his job was done.