Kraken
Kraken are legendary sea monsters of mammoth size. They are said to live off the coast of Norway and Iceland. Because of the sheer size and the horrifying appearance of the Kraken it has been used in many fictional works, because of this they are one of the more popular ocean dwelling monsters.
It is possible that the legend its self may have originated from sightings of giant squid. Giant Squid are scientifically proven to exists and grow up to 50 feet in length, including the tentacles. Giant Squid generally live at extreme depths however they have been known to rise to the surface and even attack ships. Early settlers and voyagers attacked by these beats could have exaggerated the size of the creatures thereby producing the Kraken myth.
Although Krakens never actually make an appearance in the Norse Sagas, similar monsters known as the Hafgufa and the Lyngbakr do exist. Both of these creatures are described as a giant sea spider. These could very well be the Kraken as the Norse may not of known what octopi and squid looked like at the time. Kraken were also recorded by Erik Pontoppidan, in a book called "Natural History of Norway". In his book he describes the Kraken as an animal which is the size of an entire island. He proposed that a young specimen of the monster had washed up on shore mangled and dead at Alstahaug. He argued that sailors should not fear the creature it's self but rather the whirlpools it left behind after quickly submerging back into the deep. That being said Pontoppidan also described the monstrous force the Kraken possessed. He said that the beast would be able to take the largest warship and drag it down into the deep abyss given the opportunity.
Since the beginning of the 18th century, kraken have been recorded in many different ways, primarily they are known to be large octopus like creatures, this is why many people think that Pontoppidan's Kraken is based off the sailors stories of giant squid. Earlier reports of the creature however depict it as more crab like and state that it moves around much like a whale rather than an octopus or a squid. Later editions of Pontoppidan's "Natural History of Norway" have since removed the Kraken and sightings of beast have diminished. If the creature is real it has most likely receded to deeper water or has become endangered as a species.
Whether or not this creature is fact or fiction, it truly has shaped much of our mythological history. Tales of the Kraken have been told and retold throughout the ages and it finds itself in new work every year. If we do ever find this mysterious beast we will have to rewrite the history books for sure.