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 deep in the Eastern Ocean of Narnia. Very little is known about them, although many have foolishly wanted to see one. Based on one's encounter with the Dawn Treader, they are hostile to seafarers, but also seem 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. The sea serpent then looked at its coils, apparently confused by the escape of the Dawn Treader, and dove back into the sea.