This is how I remember ninetails on our server (I'm a day 1 player). 1st time hp = 1k 2nd time hp = 5k 3rd time hp = 25k 4th time hp = 125k 5th time hp = 625k 6th time hp = 3.125m
I hope this info helps you.
That's about right.
Ninetails HP will gradually scale up with time. It's not a bug, this is intended.