Sea Serpent

Sea Serpents were massive sea-dwelling creatures that lived in the seas in Narnia. Sea Serpents seem to be generally hostile to seafarers. The Dawn Treader and it's crew encountered a sea serpent on their voyage in search of the Seven Lost Lords of Narnia. It coils itself around the ship, seemingly in an attempt to break it in two (otherwise it may have just entangled itself with the ship in an exceedingly idiotic fashion). Due to the serpent's extremely strong hide, swords and arrows were useless against it, and only by following Reepicheep's advice to push the coiled body over the side of the ship, were they saved.

In the movie, a sea serpent is shown to be Edmund's worst fear. When Lord Rhoop tells them not to think of their worst fear, Edmund unintentionally thinks of a serpent which proceeds to attack the boat. Eustace (in dragon form) engages the serpent, attempting to maul it. The sea creature clamps its jaws shut on Eustace, and drags him beneath the water. Eustace eventually makes it to a rock formation and fends off the beast with a blast of fire to its mouth. The serpent turns its attention back to the boat. At Edmund's advice, Caspian steers the ship toward the rocks. The sea serpent is rammed onto the rocks but survives, splitting its body into a cobra-like frill covered in centipede legs. The crew fire multiple harpoons at the monster's underside, to restrain it, and the creature is finally destroyed when Eustace reunites the swords. This causes Edmund's sword to glow, and, as the serpent lunges at him, he rams the sword into the roof of its mouth. Instantly, bolts of energy illuminate the beast, and it sinks beneath the waves.

Presumably, Sea Serpents resided in the Silver Sea, near Aslan's Country. It was also one of three sea beasts that Sea People were afraid of.