Board Thread:General Discussion/@comment-4317207-20130313091731/@comment-2211792-20130313162454

It could be anything. A regular Hollow (maybe as strong as Metastacia, explaining how it could have given Isshin a hard time), an Adjuchas or a Vasto Lorde (the latter would be very difficult to explain, as I don't see what use Aizen might have for it other than turning it into an Espada-level Arrancar. And I'm sure the Hollow was sent there by Aizen), or even a fully-Hollowfied Visored. The likeliest outcome of this I can see is Masaki absorbing the Hollow just like Quilge Opie absorbed Ayon. That would explain why it looks so similar to Hichigo's super-Hollow form.