The Sea Serpent has a long, purple body with an orange moustache and hair.
The Sea Serpent made its first and final appearance in Friendship is Magic, Part 2, where half of his moustache was sliced off by Nightmare Moon.
In order to cross a lake, the main six had to deal with him, as he was throwing a tantrum and blocking the route to the side. All of the ponies except Rarity thought he made a big deal out of nothing, although later on Rarity cut her tail and used her magic to stitch her tail with his cut moustache. As a sign of gratitude, the Sea Serpent allowed the main six to cross the river, even allowing them to use his body as a bridge.
The Sea Serpent lives in Everfree Forest, in a lake in the middle of the woods.