SPOILERS: It's a helmet (with a retractable tongue).  From the videos I've seen, you can see part of his face behind the tongue in an appearance early-ish, and just before his boss battle then his entire face is revealed (although it's still somewhat strange).  The whole thing seems to be part of his armour and parts of it can be unfolded and hidden at will.  (In addition, it might be interesting to add Zant's lunatic behaviour when you meet him as a boss, in contrast to his more serious depiction earlier - I haven't played the game, but possibly it's because Ganondorf's left him to return to Hyrule personally?)