Theory: Nagakabouros a major Void Beast

Step One: Look up the symbol of the void for Runeterra

Step Two: Look up "The Yellow Sign" symbol and see that it references Hastur

Step Three: Look up Hastur. It looks like a many tentacle beast, siniliar to a kraken

Step Four: Consider the following mentions from the lore;

Nagakabouros resides near Bligewater. The locals of the area worship her has a deity.

Look at the map of Runeterra, Bilgewater is really close to the mainland city of Icathia. In the lore there is a strong void prescence in the city which transforms people (Kassassidin and Malzahar).

Also near this area is Bandle City, the home of Yordles. Yordles are said to see ways of traveling Runeterra that others can not fathom. This may be referencing the void and other dimensions. Consider that Lulu went missing and returned completely out of touch with reality.

The vast majority of strange occurrences occur in the area between icathia and bilgewater. (Harrowing)

Consider Illaoi, the Kraken Priestess. Her release skin was void bringer illaoi. This may hint to the true nature of Nagakabouros.

Feel free to add any other suggestions to this lore theory or add evidence that can debunk it.