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. It came up on what they first took to be a huge neck, but as more and more of it emerged everyone knew that this was not its neck but its body and that at last they were seeing what so many people have foolishly wanted to see — the great Sea Serpent."

- (Chapter 8)

The Sea Serpent was a massive, serpentine creature that lived deep in the Eastern Ocean of Narnia. It is implied that there is only one such creature. Very little is known about it, although many have foolishly wanted to see it. It is hostile to seafarers, but also seems to lack any significant intelligence.

Dawn Treader Encounter
The crew of the Dawn Treader encountered a Sea Serpent on their voyage, somewhere between Burnt Island and Deathwater Island. After rising out of the sea, 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 against it. Eustace broke Caspian's second-best sword by hacking in vain at the beast. Only by following Reepicheep's advice to push the coils over the stern of the ship was the crew able to escape catastrophe; the only damage incurred was the loss of the decorative dragon-tail carving at the stern. Lucy thought that she saw a look of idiotic satisfaction on the creature's face, and she thought that it must be very stupid, for instead of turning around the pursuing the ship, it nosed along its coils as if expecting to find the wreckage of the ship there. Thus the Dawn Treader was able to escape.