I'm passionated about legends and myths from all over the world, with a particular regard for sea creatures and legends surrounding them ( also for cinematographic reasons in recent years, thanks to Luca and, obviously, Ruby Gillman Teenage Kraken - I have also heard about Netflix's The Sea Beast but I haven't seen it yet): in my native Italy, Liguria ( where Luca is set, inspired by local legends) is a real haven for sea monster legends, from the soul of the cruel French pirate captain Etienne Toutsaints who took a sea monster shape in order to guard his treasure to a legendary Hydra who attacked sailors ( Sam would be delighted), to many other sea serpent or dragon-like creatures ( including the good sea dragon Meschino, who now is the cliff of Panea, overlooking the sea), but there is one in particular that could match our movie, set in the coastal village of Tellaro.
It is told that, in the year 1660, during a particularly stormy night, the Saracine pirates attacked the town: unfortunately, the sentinel overlooking the surroundings from the church of San Giorgio ( the patron saint of Genoa, the local capital, and of Liguria in general, known for fighting a ferocious dragon) was asleep and so was unable to raise the alarm... until a giant octopus emerged from the sea and rang the bells of the church, calling on all the inhabitants. Some curious people, wishing to know how the bells could ring without their lookout sentinel, went to the top of the bell tower and found the octopus still ringing them: the people were grateful to the animal and a stone plaque outside the church still commemorates its heroism. Although, in Ligurian legends, many sea creatures are fierce monsters that devour sailors and wreck ships ( does that remind you of something in Ruby Gillman?), this one ( as well as the aforementioned Meschino) is portrayed as a benevolent character, maybe also as a metaphor of the endless bond among the Ligurian people and their sea.
Well, certainly that fits with the description of Krakens in Ruby Gillman... and maybe, in Ruby's world, this creature could well be a Kraken, a noble protector of the ocean, maybe even an ancestress of Ruby herself, who can say? Certanly, Italy has a good share of sea legends that could very well fit into Ruby's universe ( not only in Liguria: the old name of the city of Naples, in southern Italy, Parthenope, is said to come from a mermaid of the same name - for the joy of Nerissa, even - and the Strait of Messina, separating Sicily from mainland Italy, was the home of Scylla and Charybdis, two ship-sinking sea creatures also mentioned in Homer's Odissey)... if you go to a seaside place in Italy ( or if you already live there), try to check some of these stories, in the local library or elsewhere.
If you would like to dive deeper in this realm of sea monster stories, here is a link for a National Geographic Italy article citing some of them ( in Italian):