[Orochimaru-basher]

Maybe it's a step above a vasto lorde? Just like Uliqy's second form, no-one really know about them before. It looked like a physical arrancar release (mask covers more of face, tattoo's cover body and hole more visible etc.) but he still maintained his bankai too, I reckon that his inner hollow is just blending with Ichigo's massive amount of reatsu (hence the colour change to black and red earlier in the series), and it is constantly evolving because it is part of ichigo and he gets more powerful after each fight. Because he is fighting much tougher opponents, it has to adapt to be a step above a vasto lorde, or at least somewhere near its level.