A fox (or fox girl) with nine tails. This is stated in Japanese lore to be the pinnacle of age and power for a kitsune, and takes at least 800 years to attain, 100 years for each extra tail the kitsune grows as their power and age increases. Kyuubi are supposedly near-godlike in power. The Korean equivalent is called a gumiho.
This tag implicates kitsune and multiple_tails (learn more).