Onamazu are a type of magical catfish from Japanese mythology and folklore, they are among the more famous of the myriad assortment of demons and spirits native to such lore (collectively known as yokai).
Onamazu are blamed for earthquakes and tsunamis, due to their habit of thrashing much like their smaller cousins - their gigantic size makes them a danger to humans, other yokai and even some of the gods.
In ancient Japan the idea of earthquakes was originally blamed on large dragons but over time the Onamazu began to claim responsibility for such destruction as people envisioned the gigantic catfish as dangerous monsters of gigantic scale that would cause devastation every time they woke and thrashed around.
The reason behind this myth seemed to originate in a belief that catfish could predict earthquakes, thrashing violently before such distasters - this stuck in the imagination and thus stories spread of the Onamazu and its earthquake-inducing movements.
One particular legend states that the deity known as Takemikazuchi once took it upon himself to slay a Onamazu to stop its destructive ways, piercing its skull and tail with magic stones that remain in his shrine : it was believed that any earthquakes that occured during the 10th month of the lunar calendar were the result of Takemikazuchi's absence from his shrine (hence the month being refered to as the "godless month", due to a belief all gods left to Izumo for that month).