Ryujin
Japanese Sea Dragon
The Japanese sea dragon Ryujin - also known as Rinjin - is the god / king of the sea. The name comes from ryu = dragon and jin = god / king.
Ryujin lives in a palace under the sea known as Ryugu or the "Dragon's Court". He is attended by creatures of the sea with turtles often considered his messengers.
Ryujin controls the tides using magical tidal jewels. These were used by the Empress Jingo to aid her in an invasion of Korea by beaching the enemy fleet.
It is sometimes said that Ryujin can change shape into that of a human. According to one legend, in this form he was the father of Otohime, the "Luminous Jewel". The beautiful Otohime married prince Hoori and bore his children before she changed into a dragon. The first Emperor of Japan, Jimmu, was Otohime and Hoori's grandchild, thus making Ryujin an ancestor of the entire Japanese dynasty.
How The Jellyfish Lost Its Bones
One of the most famous legends surrounding Ryujin involves the poor jellyfish.
It is said that Ryujin wanted to eat the liver of the monky (yeeuch!). He dispatched the jellyfish to get the liver for him, but the monkey tricked it. Ryujin was so annoyed by the failure of the jellyfish that he flew into a rage and smashed it against a rock, breaking its bones apart. Since that day the jellyfish has been its current shape.
As with most legends, there are many variations of the basic story. For example, sometimes the hapless servant is the octopus rather than the jellyfish.