Saiyan

Saiyans were a tremendously powerful species with enormous ki potential. They were by-and-large eradicated by the warlord Frieza, leaving only a handful of Saiyans left in the universe. Their species are capable of undergoing a multitude of transformations.

Historical Background
It is debated as to whether the Saiyan species were in fact indigenous to Planet Vegeta / Planet Plant or if they immigrated their somehow, but it is known that they shared the world with an advanced civilization of tiny humanoids known as Tuffles. While some state that the Tuffles were friendly towards the barbaric Saiyans, others state that they instead treated them like slaves. Either way, the Saiyans eventually stormed the Tuffle cities many times in several weak attempts to overthrow their tiny neighbors; however, while the Saiyans were certainly bigger and much stronger, they were in far fewer numbers than the technologically advanced Tuffles, which included but was not limited to equipment that allowed them to scan the Saiyans to read their power levels.

Unfortunately for the Tuffles, however, the last of these attacks took place the night of their planet's full moon, an event that came about only once every twenty years. Each and every Saiyan then transformed into Oozarus and utterly annihilated the entirety of the Tuffle race. With that, King Vegeta renamed the planet after himself, and Planet Plant became Planet Vegeta.

The Saiyans wasted little time moving into the Tuffle ruins, and used their technologies to build a rocket ship to travel beyond their planet's atmosphere. They eventually arrived at a nearby planet called Arcose, home of the Arcosians. Due to their planet having become a wasteland yet with access to both monetary funds and technologies, the Arcosians hired a contingent of Saiyans to overtake a planet for them with the promise that they would share some of their technology with the brutally uplifted species. It seems likely that they accepted. The sudden wealth of technological knowledge made them somewhat smarter and less barbaric, but having failed to produced the technologies themselves left them a bloodthirsty and destructive species. While the eventual fate of the Arcosians is unknown, the Saiyans themselves were eventually discovered by the warlord Frieza and his empire. Frieza noticed the fighting potential of the tailed race, and like many races before them, he drafted them as soldiers into his military.

Trivia

 * Events portrayed in the Dragon Ball GT series is considered non-canon, and as such, so is the Super Saiyan 4 form (although Golden Oozarus do exist, as the original Super Saiyan could only retain his self-control in this form).
 * According to the timeline of the upcoming Dragon Ball Online, the Saiyan race has gone extinct by Age 1000 (216 years after the end of Dragon Ball Z).