Well, why did we not hear of any legends about Super Saiyan Berserker in Universe 7? (Out-of-universe it had not been conceptualized yet) In-universe it was probably forgotten about.
I had an idea of where it could have come into play in Saiyan history. On Planet Vegeta a long time ago, there was the presence of evil Saiyans emerging. A planetary conflict caused the good Saiyans of Planet Vegeta to face off against the evil Saiyans. Eventually, the evil Saiyans win and evolve into the Saiyans we saw under Frieza's rule.
During this planetary war, a Saiyan transformed into the first Super Saiyan God and championed the good Saiyans. What I am proposing is that the evil Saiyans had a champion of their own -- a Saiyan who transformed into Super Saiyan Berserker. These two Saiyans led the opposing factions against each other -- red versus green.
Now, why did we not hear of Super Saiyan Berserker then? Well, considering there is no given time that this event occurred, we may estimate it was a very long time ago. Legends change and evolve, and since no one had seen the green-haired Saiyan, it could have become the legend of Super Saiyan 1. The original Super Saiyan tried to go green, but was unable to and resulted in the yellow-haired form we know -- he used anger just like Super Saiyan Berserker, but it was not channeled right. This legend of a yellow-haired Saiyan became more prevalent and was passed on to Vegeta.
Additionally, why was it that the tale of the Super Saiyan Berserker changed, evolved, and was passed on, yet, no one had heard of the tale of Super Saiyan God. As we saw, the good Saiyans led by the Super Saiyan God lost the fight, and history is told by the victors. The evil Saiyans won the civil war and would want the tale of their hero (Super Saiyan Berserker) to be passed on, and not the tale of the good Saiyans' hero. New generations of Saiyans learning of the power of Super Saiyan God could potentially lead to a coup, so the evil Saiyans, who were the victors, made sure the form stayed a secret.
What do you all think of this? I really fucking love this idea. It puts so much more depth into Saiyan history and the forms themselves. Also, imagining this war is very fulfilling.
