While these days Namekians seem to be extremely outclassed by Saiyans in the world of Dragon Ball, this was not always the case. At one point, King Piccolo was one of the most powerful threats in the series, and even Piccolo Jr. was a major reason that Raditz was even able to be defeated in the Saiyan Saga.
However, there was once a Namekian so powerful it was said that he would have been powerful enough to not only defeat Frieza but was even stronger than Super Buu. While he was rarely ever seen on screen outside of small flashbacks, the Nameless Namekian is a character that has deeply shaped the world of Dragon Ball behind the scenes.
The History Of The Nameless Namekian - Katas
Oddly enough, the Nameless Namekian actually does have a name — Katas. His name was revealed in Dragon Ball Xenoverse 2 to be Katas in a quiz, which is the same name as his father. The games also show off an Ancient Katas Battle Costume which shows what Namekians of yore may have worn into battle.
The major of information on Katas comes from small flashbacks in the Dragon Ball anime. He was originally born in Age 242 just before a natural disaster left his home planet with very few other Namekians alive at all, with Grand Elder Guru and Lord Slug being some of the only other notable survivors. Very similar to Superman, Katas' father launched him on a spaceship away from the planet in hopes of saving his life. He eventually landed on Earth, spending the majority of his life living in his ship in Yunzabit Heights hoping someone would come looking for him.
After no one came, he eventually took to exploring the world, but during this time, he saw much violence and hatred that tainted his heart with evil. When he tried to become the new Guardian of Earth, the previous Guardian asked him to cleanse his soul of such evil. By doing so, he separated himself into Kami and King Piccolo and was able to defeat Garlic and become the new guardian.
This did come at a significant cost. Not only was King Piccolo unleashed onto the world, only to be heroically sealed away by Master Mutaito, but Kami suffered from an extreme loss in power level as well. While King Piccolo was eventually killed by Goku in a major fight, he was able to spawn his own offspring in Piccolo Jr. who was evil himself for some time before eventually growing kinder over the series.
While not considered by most to be canon, those who have watched Dragon Ball GT may also know that the Nameless Namekian also created the Black Star Dragon Balls. Ultimate Shenron is even more powerful than any of the other dragons seen in the series at the time, but getting a wish from him comes at a terrible cost. Anyone who wishes on the Black Star Dragon Balls must return them to the planet where the wish took place in a year's time or the planet will explode. These are used to not only make Goku turn back into a child but to restore Planet Vegeta as well.
Is Piccolo Jr. The Nameless Namekian?
There is much debate over if Piccolo Jr. is now truly the Nameless Namekian after fusing with Kami. Theoretically, this is combining both King Piccolo together (through his offspring) with Kami, and should result in one of the most powerful fighters in the entire universe. This doesn't seem to be the case for several reasons. Easily enough to start, he lacks the kind of power that Namekians claimed could have taken on Frieza at full power as is hinted at by Nail. In the manga, Grand Elder Guru even believes that only a Super Saiyan could have defeated Katas in his original form. While Piccolo Jr. is able to take on Frost in Dragon Ball Super, that isn't exactly the same.
While not canon, several of the video games suggest that if Piccolo Jr. with Kami as a part of him chose to fuse with King Piccolo, he would be able to have the full power of the Nameless Namekian once more. This would be quite the extreme power boost as it's hinted in Supersonic Warriors that this would have made Piccolo be as powerful as Super Buu. While this would seem tempting, this would potentially allow evil back into his heart resulting in unknown consequences. In Budokai Tenkaichi 3, there is a special dialogue where King Piccolo suggests that he and Piccolo Jr. actually fuse together. While he initially refuses, it's possible to get him to agree to the fusion but only if he gets to remain the dominant personality similar to how he did so with Kami.
So while this is never explicitly mentioned in the anime, it's unlikely that he truly has the kind of power the original Nameless Namekian had, though he might come close, especially considering that he is also fused with Nail.