The Dragon Ball franchise is notorious for giving its characters an excess of new forms and powers to keep its constant fight scenes interesting. This has given rise to numerous fake forms created by imaginative fans around the world.
One such form is Super Saiyan 10, the potential apex of a Saiyan's power. Appearing in a notable online fan-made continuation of the anime, Dragon Ball Xz, this form looks radically different from the Saiyan transformations before it. Imbued with massive power and usually boasting a beast-like look, this form's ridiculous concept might just be too out there to ever be canon.
Super Saiyan 10's fan-made nature was obvious from the jump, though that didn't keep it from making the rounds in the fandom. Fans quickly began using the form in their various fan fiction, eventually creating a backstory and scenario to explain the over-the-top "new" transformation. As its name would suggest, it is a form far advanced in the numeric line of Super Saiyan forms, with the power to back it up. In fact, its raw strength is beaten out only by the Super Saiyan 100 and Super Saiyan 200 forms, which are equally as non-canon.
One fan-series that prominently featured the form was Dragon Ball Xz, which took place after the events of Dragon Ball GT. During a fight with new villain Dark Trunks, Goku's power, and intensity surpass any of his previous forms. He breaks through even his Enraged Saiyan mode, leading to him finally transforming into Super Saiyan 10. This form looks like a miniature version of the Oozaru Great Ape, but shares traits with the original Super Saiyan forms.
The hair, which is longer than the first two Super Saiyan forms, and the body and tail become a chalky white color. Likewise, fur covers the entire body, unlike the halfway look of GT's Super Saiyan 4. The face also takes on a more primitive, slightly bestial look to match the rest of the body, which is now considerably more ripped. No explanation is given for this Oozaru-esque mode, though it's likely meant to express the total realization of a Saiyan's raw power.
This form grants the user powers like telekinesis and powered-up exclusive attacks like the Stellar Kamehameha, which allows Goku to use the power of the stars themselves. Another interesting ability is the power to manifest objects out of nowhere which, when combined with the fact the form is 10 times stronger than Super Saiyan 9, makes it incredibly lethal. The user's eyes also change color depending on their magic level. Other uses of hypothetical Super Saiyan 10 forms kept the white color scheme for the hair and tail but otherwise kept a normal, non-bestial design. The bestial look is the most popular, however, has been the design that started the rumors in the first place.
Quite simply, the form will never be even close to canon because of its ridiculously complex fan-made origins. Obviously, the form comes after 6 other numbered Super Saiyan forms, one of which (Super Saiyan 4) is itself no longer canon. The fact that Super Saiyan 4 was invented for the franchise's most divisive entry doesn't help. The official manga would have to introduce all of these numbered forms before ever introducing the admittedly ridiculous level of Super Saiyan 10.
The discrepancy of its "official" design would also present another stumbling block in bringing it to the mainstream continuity. The design that simply gives Goku white hair and a white tail, without the added bulkiness would honestly be far too mundane, especially given that this is supposed to be one of the most powerful Saiyan transformations.
The official Super Saiyan transformations have currently eschewed their original numeric line in favor of the Super Saiyan God forms. These forms, instead of numbers, represent power through their color schemes. The Super Saiyan 10 form was somewhat similar to this in regards to the eyes, while still keeping the numbered line going. Due to the franchise narratively leaving that line behind, perhaps it's best that it does the same with the idea of Super Saiyan 10 and leave it to the fans.