Kraken
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"Release the Kraken!"
- – Zeus, signing the death sentence of a kingdom
The Kraken is a monster from European mythology, presumably Nordic, and is described as a sea-going creature of such enormous size that when it comes to the surface to doze and bask in the sunlight, sailors have been known to mistake it for an island and walk around on its back, getting a hell of a surprise when it inevitable decides to submerge again.
In the original myths, the kraken is never really described beyond being a bloody huge thing living deep under the water. In pop culture, however, it has become synonymous with an incredibly huge squid, though it's unclear if this is because of Dungeons & Dragons doing it or the other way around.
D&D
In D&D, the kraken is typically presented as an enormous aberration or magical beast, in the form of a ridiculously huge squid with genius-level intelligence, powerful magical abilities, and an even bigger ego. Ironically, despite 4th edition's terrible reputation, it actually preserved the traditional kraken, and even gave it a nastier relative that hunted the Astral Sea. It was 5th edition that broke with tradition, portraying its kraken as a deformed fish-frog-lizard thing with tentacles, though it's still an egotistical super-genius with a spate of nasty legendary actions.
Birthright had the Kraken as one of the unique awnsheigh that bedevil Cerillia, and, atypically for one of the animalistic ones, also a super-smart genius. It is terrifying, given its massive size in a relatively-grounded and realistic campaign setting, and no one knows what the actual fuck it is, what it's doing, or why it picked the former Great Bay, now the Krakennauricht as its territory. Maybe it was one of the mounts Azrai rode across the sea, maybe it was a creature from the elemental plane of water the evil god corrupted for some inscrutable purpose. Either way, it's the biggest monster in the world... and it may have islanders performing human sacrifices and worshipping it. It's hard to tell; they're all understandably more scared of the Kraken than they are of anyone trying to contact them, and while they've been observed they're never anywhere to be found when brave ships make landfall.
MGE
In the Monster Girl Encyclopedia, the kraken is a giant squid-girl, a relative of the scylla. Taking the form of a taller-than-average and hugely busty white-skinned woman with ten squid-like tentacles, they are mostly shy and retiring - but when they go hunting for a boyfriend, they have been known to sink whole ships to snatch up the man they like, leaving the rest as prey for the swarms of aquatic mamono who flock to them when they go husband-hunting. They have the ability to spit a magical ink that consumes light, creating magical patches of darkness.
40k
Warhammer 40k has Hive Fleet Kraken, a large fleet of Tyranid organisms noted for encircling foes at every level. There are also proper krakens found on Fenris, although they are largely suspected to be of Tyranid origin.