User blog comment:D.Dragon123/Why Haoshoku haki is useless./@comment-78.216.252.84-20130115194558/@comment-78.216.252.84-20130115203135

I don't know, really. I just thought of something possible in the OP universe that is powerful enough.

I thought of that because the four users we know were able to pet powerful beasts or animals and the fact Raileigh had Luffy pet powerful beasts to train him with this haki.