WHO IS THE SAIYAN NAMED "YAMOSHI"?

Yamoshi. Yamoshi is saiyan who was super saiyan long before Goku. We'll know how Goku became super saiyan. He was fighting with Freiza and he killed Krillin that triggered him to became Super saiyan. However we don't know How he became Super saiyan. After his dead he was known as Super saiyan god (with ritual way). He was known as a myth until Goku became super saiyan.

He was never showed in dragon ball so in his silhouette, he more looked like Vegeta even the clothing matched. He was shown while he became super saiyan he looked skinnier. So he didn't looked buff like Goku and Vegeta. 

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Yamoshi is a Saiyan with a righteous heart who existed long before Planet Vegeta was the Saiyan's planet. Alongside his five comrades, Yamoshi starts a rebellion against the evil Saiyans. He is cornered and becomes a Super Saiyan, but is still defeated as he runs out of energy. A Namekian elder sympathized with Yamoshi's spirit, and so recorded the information about him in the Namekian Book of Legends.

Vegeta states that the last Super Saiyan to appear was a thousand years prior (three thousand in the Ocean dub) to the beginning of Dragon Ball Z. The original legend of the Super Saiyan was more about his power than the Saiyan himself.

When the Dragon Ball Z anime was released, a filler scene in the episode "Goku's New Power" was added showing the Super Saiyan as what looks like a Great Ape with yellowish colored fur instead of the standard brown, maybe it was Golden great ape. In the filler visual, the Original Super Saiyan is shown attacking people around him.

The Super Saiyan form is believed to be a myth, until Vegeta begins to suspect that Goku is a Super Saiyan after seeing Goku easily defeating the Ginyu Force on Namek, thus witnessing his drastic increase in power in a short period of time. Vegeta's suspicions are not confirmed until later, when Goku transforms against Frieza, and states himself that he is a Super Saiyan.

According to Shenron's story about Yamoshi from the Namekian Book of Legends, six righteous Saiyans came together on Planet Sadala to transform one into the Super Saiyan God. However, while the Super Saiyan God overwhelms the evil Saiyans, his power vanishes as quickly as it came, and as such, he and his comrades are wiped out.

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Beerus saw Yamoshi's spirit in his prophetic dream. Many years later while trying to remember the warrior he saw in his dreams, Beerus saw Yamoshi's spirit as he destroyed a planet. Later, after being asked by Goku how to attain the Super Saiyan God transformation, Shenron - with his knowledge from the Namekian Book of Legends - tells the Dragon Team, Beerus, and Whis the story of the Super Saiyan God. Yamoshi's spirit appears during the Super Saiyan God ritual, turning Goku into a Super Saiyan God to battle Beerus.

When Super Saiyan God Goku is battling Beerus and believes he has reached the limit of Super Saiyan God's power and is going to lose, the "Super Saiyan God power" (Yamoshi) speaks to Goku and tells him to keep on pushing - allowing Goku to tap into more power and continue the battle.

A mobile Game Dragon ball Legends. Shallot and Giblet (DBL original character) are ancient living saiyans living in a time period after the demise of original Super saiyan god, with the righteous twin brothers battling against the Evil Ancient Saiyans of their time. Giblet explained to Shallot that the power they needed to obtain to defeat the evil Saiyans was Super Saiyan God, explaining to Shallot that they both possess the potential to become Super Saiyan Gods, due to possessing the same righteous will, declaring them the rightful successors due to the blood of the original Super Saiyan God running through their veins, indicating they may be descended from Yamoshi and seek to carry on his legacy of opposing the evil Saiyans. However, as Saiyans of the modern era are descendants of the evil Saiyans who killed Yamoshi, Giblet sees his mission as to kill all modern Saiyans to restore the true Saiyan way.