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
This monster 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 Voyage of the Dawn Treader
The Silver Chair
The Horse and his Boy