Wdym original work? Like the actual fairy tale this story is based on?
Yes, as far as I know, 90% of the story was Goku, 9% is Hakkai, 0.9% for Sha Gojo, and last 0.1% for the Horse (the horse was skipped for most JP adaptation as far as I know of).
You got one page backstory that he was, just like Hakkai, a Heaven's general, but with a lot less important role. He broke some important vase, banished to be a river demon, and will be able to redeem himself after helping Sanzang complete his pilgrimage. And that's it. He beats a few (literally, not figuratively) low level demons along way and be errand boy sometimes, but mostly he'll be the Yamucha to show that the opponent is so strong he can't do a thing to them.