The kyuubi no kitsune (literally 'the nine-tailed fox') is a powerful youkai (Japanese word for spiritual and otherwise supernatural creatures in folklore) commonly seen in Japan, China, and Korea. A normal fox is said to be able to turn into one, by gaining a tail each century of living and spiritual training.
Records of the nine-tailed fox can be seen first in China. It is a benevolent, if not outright divine creature, and anyone who can encounter one and live to tell the tale is destined to become a lord or even the emperor. In Korea these creatures wish to become human, often by stealing the life force of human males by transforming into a woman and seducing them. Common folklore depicts them as more violent creatures that actively prey on male human livers and corpses. In Japan, however, nine-tailed foxes are often depicted as evil and massively powerful entities, their power nearly equalling that of deities.