Kraken

...and while the sailors were resting on the ship, a gigantic octopus appeared on the horizon. Kraken! was shouted, and while we off the ship jumped, most of the med slowly drowned. The thing was big, appeared from nothing. Huge tentacles, had long teeth... Freido Mari

Known from pop culture and medieval times as a formidable octopod cephalopod killer, wasn't written down as an actual thing until Jules Verne did. Yudonia, being an island in the middle of a ghost zone of the most transited sea, saw many ships supposedly disappear by this mythical creature.

Origin

It is thought that the Kraken, as any other mythical creature, was originated as a made-up history on the medieval ages, but it started to spread as one of the very few possible explanations for the disappearance of numerous vessels, while in reality a simple storm would have been able to destroy a big ship.

Historical basis

There have not yet been found any proof of a real animal or vessel that could have been misinterpreted as a big animal of such magnitude. There has been speculations about the possibility of Blue Whales' fins to had been misunderstood as cephalopod tentacles, but it has been pointed out as a very unlikely situation.
Most likely the mith was just made up by some drunk sailors on their way home or by a family member of a sailor lost to the sea, trying to get answers or at least an explanation of the loss of a ship, which, in reality, was very common by those days.

Expansion

The mith probably managed to reach all ports within the west, as it is very well known in pretty much all of Atlantic countries nowadays as an old mith.

Image: Pen and wash drawing by malacologist Pierre Dénys de Montfort, 1801, from the descriptions of French sailors reportedly attacked by such a creature off the coast of Angola. Public domain - Wikipedia