the rikudou sennin has it because long ago he fought the ten tails and sealed it inside himself then became the first jinchuriki
No because he's not the 9 tails... he has the curse mark and there are no tails for that.
Scorpion
The Ten Tails makes its first appearance in Naruto Shippuden Episode 329, titled "Two-Man Team." This episode marks the beginning of the Fourth Shinobi World War arc, where the Ten Tails is revived by Obito Uchiha and Madara Uchiha. The Ten Tails plays a significant role in the war as a powerful and destructive force that threatens the entire ninja world.
the nine tailed fox demon is the strongest now, but in manga Madara Uchiha aka Tobi says that all the tailed beasts were once the chakra of the ten tailed beast called juubi.
The phrase 'ten tails' would be 十本のしっぽ (juppon no shippo) in Japanese. However, the word十尾 (juubi) has been used in the anime 'Naruto' to refer to a demon called 'Ten Tails'. I wouldn't recommend using this word, as it could also mean 'ten small fish'.
Using senjutsu.
Probably Madara after he became the Ten-Tails Jinchuriki...
The ten tails is the mixture of all the tailed demons combined. It basically black with one red eye ball, presumably the sharingan ultimate form. No one is a ten tails, though the chances of Naruto becoming ten tails is likely
Naruto isn't or hasn't been an Hokage, and Juubi (ten tails) is currently sealed to the moon, as stated by Madara.
ye
Yes, there was a ten jinchuuriki. The Sage of the Six Paths had split the chakra of the ten-tailed jinchuuriki and spread them throughout the land, creating the tailed-beasts (tails ranging from one to nine tails, each beast consisting of the ten jinchuuriki's chakra). The Sage of the Six Paths then sealed away the body and final tail of the ten jinchuuriki away in what later became the Moon.
The ten tails begins to revive in episode 341 of shippuden and naruto tries to stop it in episode 342 of shippuden as it still starts to revive. And finnaly at episode 362 of shippuden at the end it gets revived, sorry for the late response XD.