is the nine tailed fox really a true ancient japanese legend? if so what was it about?

Yes it is based on an ancient Japanese legend, and it is apparently created by a group of really strong wizards, there were nine wizards so the beast has nine tails one for each wizards (sorcerer).

of course it is real

鬼

i'm pretty sure it's real.:/ it's been seen on more than one anime sooo...yh.

In most asian cultures, foxes are renowned shapeshifters. Both Korea and Japan have actual legends about nine-tailed foxes.
In Japan, foxes are associated with the deity of rice and fertility, Inari (an androgynous deity who takes on the form of both sexes). One with nine tails is believed to be a sign of extreme good fortune, and eating one is supposed to protect the consumer from evil spirits.

link

link

a Shinto Kami spirit and as in old tales

The Kitsune: Kyūbi no Kitsune/Huli Jing
"Kitsune" is the Japanese word for fox or fox spirit a forest spirit. It does span other regions: China, Korea...

most likely you will find your answer here:
link

and the NINE tails are the oldest and wisest stage of Kitsune.

Yes, it is from a true ancient Japanese legend. Legend has it that foxes, depending on their wisdom and age, can have a certain number of tails. The limit is 9 tails, so thats where Naruto, Pokemon, and Okami got it from. Let me know if this helps!