While he doesn’t have one in the current landscape of Dragon Ball continuity, Goku originally sported a monkey-like tail, which is actually something all Saiyans are born with. As fans quickly learned, the Saiyans’ tails offered them the ability to transform into an immensely powerful form known as the Great Ape, and the removal of one’s tail meant that Saiyan could no longer make that transformation. However, Goku’s Saiyan tail had another secret power that wasn’t as dramatic as the Great Ape transformation but still proved to be incredibly useful nonetheless. Interestingly, it's a power that fans have completely forgotten about.
When readers are first introduced to Goku in the pages of Dragon Ball Chapter 1, he is just a child wandering the woods looking for food and supplies. While he was capable enough to hold his own against any creature of the forest, Goku’s energy and power level wasn’t even remotely close to what it is today and he was lacking a number of key abilities that have become second nature to the Saiyan warrior in more recent stories (the most obvious being the fact that he couldn’t fly yet). After Goku meets Master Roshi, he is given the Flying Nimbus which is a magical cloud that allows anyone with a pure heart to ride it. Flying Nimbus made up for the fact that Goku couldn’t independently fly, but in the middle of a battle or tournament, relying on an external source for flight isn’t ideal–though Goku’s tail offered him a unique workaround for that particular problem.
In Dragon Ball Chapter 47 by Akira Toriyama, Goku is battling a disguised Roshi in a world martial arts tournament in the final fight of the competition to see who will become the world champion. While both desperately wanted the title, Roshi was even more determined to win than Goku was because Roshi wanted to teach Goku that no matter how strong he gets, there will always be someone out there who is stronger. Roshi believed this was a lesson that he hoped would inspire Goku to keep training and not slack off because it's possible that Goku would believe that he's unbeatable if he had won the tournament. So, Roshi doesn’t hold back against the Saiyan child and is actually successful in launching Goku from the ring. However, before Goku’s feet touch the ground—which would have eliminated him—he suddenly begins to fly. This is a climactic moment because flying is not something Goku should have been physically capable of doing at that point in his life.
Goku became something of a living helicopter as he spun his tail so quickly it acted as a biological rotor, lifting Goku off the ground and granting him the power of flight. This was a huge reveal as it showed, for the first time, Goku flying independently. This power helped him in the tournament and became a perfect option for transportation (under extreme circumstances) in his later adventures. The only problem is, Goku almost never used this ‘secret power’ again–making it no more than a brief blip on fans’ radar. Because readers never saw this move again, they forgot about this game-changing yet entirely underutilized ability.
This secret ability exhibited by Goku’s tail is not as reliable as the power of flight Goku gains later in the Dragon Ball saga, but it was a great resource to have at his disposal in a pinch. In this case, it helped him avoid elimination from a tournament, and in a later chapter when the Flying Nimbus is shot down by a rocket launcher, Goku still has his tail as a means to fly should he have needed it–a secret power that, because of lack of use throughout the Dragon Ball series, fans forgot it.