Sea Serpent

"Suddenly, only about the length of a cricket pitch from their port side, an appalling head reared itself out of the sea. It was all greens and vermilions with purple blotches—except where shell fish clung to it—and shaped rather like a horse's, though without ears. It had enormous eyes, eyes made for staring through the dark depths of the ocean, and a gaping mouth filled with double rows of sharp fish-like teeth."

- (Chapter 8)

Sea Serpents were massive, serpentine creatures that lived in the seas of Narnia.

They were generally hostile to seafarers, and presumably resided in the Silver Sea, near Aslan's Country. They were also one of three sea beasts that the Sea People were afraid of.

Dawn Treader Encounter
The Dawn Treader and its crew encountered a Sea Serpent on their voyage, in search of the Seven Lost Lords of Narnia. It coiled itself around the ship, apparently in an attempt to break it in two. Due to the serpent's extremely strong hide, swords and arrows were useless, and only by following Reepicheep's advice to push the coiled body over the side of the ship, were they saved with the loss of only the dragon-tail stern.

Adaptions
In the 2010 film adaption, a Sea Serpent is revealed to be one of Edmund's worst fears, and is thus encountered at Dark Island.

When Lord Rhoop tells them not to think of their worst fear, Edmund unintentionally thinks of a serpent, which the Green Mist feeds off, takes the form of, and then proceeds to attack the ship as. Eustace (in dragon form) engaged the serpent, attempting to maul it. The sea creature clamped its jaws on him, and dragged him beneath the water.

It then slammed Eustace into a rock formation, but the beast was fended off with a blast of fire from his mouth. After Eustace was injured and fled, thanks inadvertently by Lord Rhoop who struck him with his sword, the serpent turned its attention back to the ship, but at Edmund's advice, Caspian steered the ship toward the rocks, ramming the beast onto the rocks. The serpent survived, though, and proceeded to split its body into a cobra-like frill, covered in centipede, leg-like claws.

The crew fired multiple harpoons at the monster's underside, to restrain it, until the creature was finally destroyed after Eustace reunited the Seven Swords.

This caused Peter's sword (Rhindon), which Edmund had been using, to glow, and, as the serpent lunged at him, he rammed it into the roof of its mouth. Instantly, bolts of magical power illuminated the beast, and it sunk beneath the waves, dead.

Trivia

 * The death custence of the sea serpent bears striking similarities to the death of the Basilisk from the 2002 film "Harry Potter and The Chamber of Secrets". Both were giant serpentine beings, killed by the main protagonists with a mythical sword, by stabbing the roof of their mouths from between their jaws. The major difference was that the Sea Serpent's death contains less gore, and no blood at all.