“Rabbit’s clever,” said Pooh thoughtfully. “Yes,” said Piglet, “Rabbit’s clever.” “And he has Brain.” “Yes,” said Piglet, “Rabbit has Brain.” There was a long silence. “I suppose,” said Pooh, “that’s why he never understands anything.”
The Leviathan is a great sea monster refered in the Bible. It has become synonymous with large sea monsters in general.
Moby Dick is the great white whale (and antagonist) of Herman Melville’s novel of the same name.
The Templeton Monster is from the novel The Monsters of Templeton by Lauren Groff (recommended)
Nessie is the name of the Loch Ness monster
The Kraken is a giant sea monster. Its description is found in many scandinavian texts (possibly based on the sightings of giant squids). Tennyson wrote a poem calle ‘The Kraken’, which might have inspired Jules Vernes for its giants squids monsters in 20 Thousand Leagues Under the Sea. The Kraken is often referenced in pop culture such as in the Pirates of the Caribbeans movies and The Clash of the Titans movie.
Memphré is a lake monster that lives in the Memphrémagog lake in Québec, Canada. I grew up in the town of Magog, which is right beside the lake. It’s my own lake monster :)
Charybdis is from The Odyssey. She is the daughter of Poseidon and Gaia, and used to be a beautiful naiad. She would be very loyal to her father and would ride his angry tides swallowing villages and gaining land. Zeus got angry and changed her into a monster. Most of the time now, she is represented as a giant whirlpool.
The Watcher in the Water is from The Lord of the Rings. It’s the monster that attacks the Felloship as they are about to enter the Mines of Moria.
The Hogwarts’s Squid is exactly that. It’s the squid that lives in the lake on Hogwart’s grounds and that occasionally rescues students who fall in the lake.
The Host, is the sea monster found in the South Korean movie of the same name (highly recommended)
Ogopogo is a somewhat famous Canadian lake monster that lives in the Okanagan Lake in British Columbia.
Cetus, of the constellation of the same name, was a sea monster from Ancient Greek mythology.
Hope that helps! (I found some of these things from wikipedia, than do a bit of research onmy own)