Sea-serpents, or Fish-Dragons, were a breed of Dragons mentioned fleeting in Elvish linguistic writings only. They are not mentioned in any stories. They were probably made by Morgoth in the First Age. The "Watcher in the Water" may have been a relative of these that came up the river Greyflood and got trapped in the lake by the Gates of Moria. Also, it could have been many of them. It is unknown what they look like, though it can be assumed they looked like a classic sea-serpent. Another possibility was that they were or are related to the Turtle-fish-like Fastitocalon of the Hobbit poem Fastitocalon.