Youko

Ichibi: 0+

All youko are born as ichibiyouko (一尾妖狐, "One-Tailed Mysterious Fox"). At some point during their time as an ichibiyouko, youko develop the ability to shapeshift into a human form. While this usually happens shortly before they turn 100 years old, some are able to transform when they're as young as 50 years old.  

Nibi: 100+

After turning a century old, a powerful enough youko can grow its second tail and become a nibiyouko (二尾妖狐, "Two-Tailed Mysterious Fox").  

Sanbi: 200+

After turning two centuries old, a powerful enough youko can grow its third tail and become a sanbiyouko (三尾妖狐, "Three-Tailed Mysterious Fox").  

Yonbi: 300+

After turning three centuries old, a powerful enough youko can grow its fourth tail and become a yonbiyouko (四尾妖狐, "Four-Tailed Mysterious Fox").  

Gobi: 400+

After turning four centuries old, a powerful enough youko can grow its fifth tail and become a gobiyouko (五尾妖狐, "Five-Tailed Mysterious Fox").  

Rokubi: 500+

After turning five centuries old, a powerful enough youko can grow its sixth tail and become a rokubiyouko (六尾妖狐, "Six-Tailed Mysterious Fox").  

Shichibi: 600+

After turning six centuries old, a powerful enough youko can grow its seventh tail and become a shichibiyouko (七尾妖狐, "Seven-Tailed Mysterious Fox").  

Hachibi: 700+

After turning seven centuries old, a powerful enough youko can grow its eigth tail and become a hachibiyouko (八尾妖狐, "Eight-Tailed Mysterious Fox").  

Kyubi: 800+

After turning nine centuries old, a powerful enough youko can grow its ninth tail and become a kyuubiyouko (九尾妖狐, "Nine-Tailed Mysterious Fox").  

Tenko: 1000+

Youko that can grow nine tails and live for a millenium are able to become Tenko (天狐, "Heaven Fox").