User blog comment:Uraharattack/Pernida -- A Theory/@comment-11051098-20150809032036

I think it's possible that Pernida and the Left Arm are 2 separate beings, originally.

I find it unlikely that the Left Arm of the Soul King is a Quincy, that would make the arm a basic enemy of its "owner".

Back in chapter 599, when Pernida was first shown, it was shown to have a normal sized left arm on the page where it prepared to use The Compulsory to wreck the giant soldier summoned by Shutara. After its hood was torn apart in chapter 636, someone said somewhere that Pernida could have been in its true/regular form at the time, taking on the full form of the Left Arm sometimes before the hood was torn.

I don't have any answer, I doubt anyone does at the moment except Kubo himself, but I expect the next few chapters to reveal the truth before the fight comes to an end.