Sea Serpents are a hostile species that exist in the world of Narnia. One of them (though it's actually the Green Mist in the form of a serpent) is the secondary antagonist of the 2010 Narnia movie, The Voyage of the Dawn Treader and its role is small. He appeared at the end in a huge, climactic battle.

Role in the film

One of these monsters came from the Dark Isle, and was born from Edmund's fearful thoughts of sea serpents, which the Green Mist took the form of in the Dark Isle. The sea monster attacked the ship several times, and was at first held off by the dragon-form Eustace, before it wounded him. The battle was long, with the serpent first being rammed against a rock by the ship, then escaping and wrapping itself around the ship as in the book. Eventually the conflict came down to the serpent and Edmund, who finally killed it with Peter's enchanted sword, which he drove into the roof of the monster's mouth.

           The Chronicles of Narnia Villains

The Lion, The Witch, and the Wardrobe
Jadis the White Witch | Jadis' Secret Police (Maugrim & Vardan) | Ginarrbrik | General Otmin

Prince Caspian
Miraz | Telmarines (Sopespian & Glozelle) | Nikabrik | Hag & Werewolf

The Voyage of the Dawn Treader
Green Mist | Sea Serpents

The Silver Chair
Lady of the Green Kirtle

The Horse and his Boy

The Magician's Nephew
Jadis the White Witch | Uncle Andrew

The Last Battle
Shift | Rishda the Tarkaan | Ginger | Tash