"According to legend, the last Super Saiyan could only maintain his status in the transformed state. His power was extraordinary, he had no equal in the universe. But he was too primitive to control it, his rage burned so intensely that it eventually consumed him. In the end, he was destroyed by his own power."
— Vegeta, in "Goku's New Power"
The Original Super Saiyan (先代の超サイヤ人, Sendai no Sūpā Saiya-jin; lit. "Previous Generation Super Saiyan") was an unnamed Saiyan who could transform into a Super Saiyan. He was originally thought to be nothing more than a myth.
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, stating that he had achieved a level of power so overwhelming that it could only be maintained in a transformed state. 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 a Great Ape with yellowish colored fur instead of the standard brown. It may be that he was the first Golden Great Ape, and he could not control it, which ultimately led to his demise.
The Super Saiyan was fabled to have been destroyed by his own power, and the filler visual in the anime suggests that his power destroyed the planet that he was on. Vegeta states that the Original Super Saiyan destroyed the Original Saiyan Homeworld immediately after his transformation, thus forcing the remaining Saiyan survivors to flee to Planet Plant (which was later renamed Planet Vegeta). In the series, the form is believed to be a myth, until Vegeta begins to suspect that Goku is a Super Saiyan after seeing 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 that he is a Super Saiyan.
Contrary to Bardock who transformed into a Super Saiyan on Planet Plant, only seen in this form by Plants and Chilled, the Original Super Saiyan transformed on the original Saiyan homeworld, among other Saiyans. Bardock's story in Episode of Bardock explains Frieza's fear of the Saiyans, as Chilled passed his knowledge of Super Saiyans to his descendants, while the Original Super Saiyan explains the Saiyans' knowledge of the Super Saiyan legend.