Questions & Mysteries Zunesha's crime pre-dated Joy Boy and the War.

[Dettles]

Zunesha apparently did something so horrible that he was sentenced to walk eternally. But that was over 1,000 years ago and over 100 years before Joy Boy and the War with the 20 kings.

 

[Kung-Fu Dugong]

Zunesha was born 1000+ years ago, but the crime was commited at the end of the Void Century (or during the Void Century, then Zunesha was condemned on its last year). Momonosuke especifically said in chapter #1040 that Zunesha commited a crime 800 years ago.

It was probably sentenced to walk around the seas until it died due to its partnership with Joy Boy. I can't really think of something else.