The Saiyans in Dragon Ball have the characteristic of being born with a monkey tail. The tail is there to transform into a giant battle ape. However, Son-Goku’s tail eventually stops growing altogether, and Vegeta and Son-Gohan are completely missing theirs. The manga artist Akira Toriyama explained why this is the case.
Why is the tail missing? For an interview with Akira Toriyama in the first Shonen Jump issue in 2003, fans could ask the manga artist questions. One reader wanted to know what happened to Vegeta’s tail after he was defeated on Earth.
Toriyama explains that it has to do with the power level of the Saiyans:
With the tail, one can gain enormous power instantly through the transformation into a battle ape, but the risks are equally great – one loses all strength if it gets pinned down. Once you are as strong as Vegeta and Goku, the tail is just in the way. It is assumed that the bodies of the Saiyans, who are a fighting species, decided that their tails are unnecessary appendages.
Akira Toriyama, Kazenshuu
Over time, Vegeta and Son-Goku simply became so strong that they no longer needed to transform into a battle ape.
The fact that the tail does not regrow in Son-Goten, Trunks, and Oan also has a simple reason: The tails are a recessive genetic trait and do not even form with decreasing Saiyan DNA. Goten and Trunks have human mothers, which is likely why their genes regarding the tail prevail.
There is one exception: In Dragon Ball GT, the tail is necessary to transform into Super Saiyan 4. However, this is an anime series that is not directly created by Akira Toriyama. Toriyama himself has never drawn the SSJ4 for the anime. Therefore, it is possible that the Saiyans in the GT version still rely on their tails.
Perhaps the tail will also be needed in Dragon Ball Daima:
Toriyama explains reasons for omitting the tail
Are there further reasons? With this explanation, Toriyama has clarified within the lore why the tail is missing. However, there is also a practical reason, as he explained in the Dragon Ball comic “Freeza Arc Vol. 5”. There, he was asked if Goku’s tail is a nuisance:
That’s true. (laughs) In the first sketches, Goku was originally a real ape. My editor said to me: “Without a tail, he has no distinguishing features,” so I added a tail. Drawing this tail was so annoying that I couldn’t stand it… so I immediately came up with an episode in which his tail gets cut off. (laughs)
Akira Toriyama, via Kazenshuu
Therefore, Toriyama found it annoying to have to draw the tail all the time. Since many manga artists are under pressure from deadlines, they save on everything that would take unnecessary time. This is also the reason why the hair turns blond in Super Saiyan.
According to a Daizenshuu issue (via Dragon Ball Wiki), there is even another reason why the tail no longer regrows. There is only an 8% chance that the tail will regrow, even if the Saiyan has not yet achieved a stronger form than the battle ape. For the tail to even regrow, a dangerous situation is necessary.
The only practical reason for omitting the tail remains time-saving. By the way, there is another stage that Toriyama omitted for time-saving reasons. Vegeta never transformed into a Super Saiyan 3 because filling in the hair would have taken too long: Vegeta is never seen as Super Saiyan 3 in Dragon Ball Z – There is an official reason
