Board Thread:Naruto Discussions/@comment-26084508-20150916103812/@comment-734582-20150917171159

Bambusek wrote: They are based on Bijuu from Japan mythology, so probably that's why. In terms of Narutoverse - I think the number of tails represents the beast power, at least partly. Only shukaku (Tanuki) and Kurama (Kitsune) are based on actual tailed beasts. Everything else is vaguely based upon other japanese monsters, but the whole number-of-tails motif is uniquely Kishimoto's addition.