Uchiha Madara did so that he could kill the 4th hokage (Yondaime)
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The one on his stomach was the seal used to imbue the Nine-tailed Fox demon inside him. It is a Reaper Death seal. This is a jutsu that his father, the Fourth Hokage, used to take away some of the fox's power by sealing it within Naruto.
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.
It almost did when Naruto was in 8 tailed form while fighting with God Realm Pain. Naruto ,while in 8 tailed form was uncounciusly talking with the Demon Fox. The Demon Fox persuade naruto on taking off the seal and Naruto almost take it off, that's when his father, the 4th Hokage appear as a memory and pull naruto away from the Demon Fox and tell naruto that he is his father and tell him stuff about the time when the Demon Fox attack Konoha. For more info read Naruto manga chapter 440 - 441.
Sora does not have a beast, just a portion of the Nine Tailed Demon Fox's chakra which Naruto Uzumaki absorbed back into himself.
"Kyuu" literally means nine and "bi" means tails (as in "Bijuu" aka tailed beasts) so Kyuubi literally translates as "Nine-tails" In the English manga and dubbed (dubs SUCK) anime, they refer to him as "Nine-tails." Or if you go to any translate station it pops up kyuubi meaning Naruto.