Bijuu

Bijuu are beasts with unimaginable power. Bijuus exist long, long ago. People tried to capture bijuus to use it as a military weapon. These beasts has tails. The first one is the Ichibi (one-tailed bijuu), Nibi (two-tailed bijuu) up to Kyuubi (nine-tailed bijuu). It is explained in Naruto Shippuden Episode 16.