Throughout Dragon Ball, Son-Goku can transform into several forms of Super Saiyan. In one movie, there is even a very special form that has never been seen again afterward. There is a good reason for this.
Where did the form appear? The special Super Saiyan was seen in the fourth Dragon Ball movie, originally titled Dragon Ball Z: Lord Slug. The German title Dragon Ball Z – The Movie: Super-Saiyajin Son-Goku already reveals that this is about the new form.
Goku fights against the villain Lord Slug here and initially stands no chance against him. In a moment of anger, he suddenly transforms into a kind of Super Saiyan that had never been seen up to that point.
The first time Son-Goku transforms into a Super Saiyan is in the fight against Freezer. The plot of the movie is supposed to take place before that.
The Z in Dragon Ball Z is no coincidence:
Son-Goku transforms into a special Super Saiyan
What kind of form was that? Goku’s hair stands up like a true Super Saiyan, and he receives a yellow aura. However, the blonde hair and green pupils are absent. Instead, the hair gets a reddish tint from the aura, and the pupils completely disappear.
The false Super Saiyan also differs in power level. Goku gains strength and is able to challenge Lord Slug for a while. But compared to the true Super Saiyan, he falls short in terms of strength.
Son-Goku does not have full control over this form. It seems as if he is not fully aware when he becomes the pseudo-Super Saiyan. Moreover, he is unable to defeat Lord Slug in that form. If you want to see the form for yourself, you can do so in a YouTube video by user RinneyUploadz1.
Why was it only seen once? The fourth Dragon Ball movie Dragon Ball Z: Lord Slug was released in 1991. Just 10 days later, the real Super Saiyan was revealed in the manga, this time with blonde hair. The reason for this is time savings.
The false Super Saiyan was therefore something like a teaser for the real, optimized Super Saiyan. That’s why it only appears once in the movie. There are further hints in other movies about the form, but it is never shown in all its glory there.
For the stronger, threefold Super Saiyan, there is even an original form that was never seen. Presumably, the manga artist Akira Toriyama decided against it because it does not differ enough from the previous Saiyan forms: Dragon Ball Z: The original design of Super Saiyan 3 never appeared in the anime.