This is a strange animal indeed in the folklore of Japan. It dwells in the water. It has tentacles and the stomach of a cow. It is said to have preternatural powers.
On one occasion a local official (dzongpon) is said to have decided to test its strength. He placed an elephant in the water with the creature and awaited developments. A ferocious struggle ensued between the two huge creatures, but then the elephant shambled ashore. The stewa rutu was still clinging to it. Local women poured hot sand on it and finished the beast off.
Whether there was only supposed to be one stewa rutu or more than one, I do not know, nor do I have a picture of this unusual creature.