Ninetails Youkai (jap. 妖怪) describes simply all kinds of Youkai with mystical/magical powers and those who archieved to nine tails, the maximum of tails they can archive. When such a Kitsune is born, he got only one tails like a normal fox and his magic powers are only at the bottom of what is possible.
When they grow older, around all 100 years they grow an extra tail naturally and their powers grows. Thanks to this phenomenon a tails does not only symbolizes their actual power but wisdom and life experience as well.
Sometimes its not sure if they can archieve nine tails just by growing older and they need to train themself as well, or need more understanding of whatever things. In any case its said that Youkai with nine tails can wield powers equally to gods 'or above'.
Most of the time this Youkais have always fox characteristics, no matter if they walk around in the body of a fox or as a human with foxears and tails.