If you breed two Pokemon together that will give you a child that is a Vulpix (so two Vulpix or Ninetales of opposite genders, or a female Vulpix or Ninetales with any compatible breeding partner, or any Vulpix or Ninetales and a Ditto), the resulting Vulpix will only have a chance of having it's hidden ability if one of the parent Vulpix or Ninetales had it's hidden ability. The hidden ability is only passed down.
So if neither parent Vulpix or Ninetales have their hidden ability, the chance is 0%.
If the female parent Vulpix or Ninetales has it's hidden ability, the chance is 60%.
If the male parent Vulpix or Ninetales has it's hidden ability and is bred with a Ditto, the chance is 20%.
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