Giant cuttlefish, octopus, or squid, perhaps?
A cuttlefish:
(http://www.oceanwideimages.com/images/14353/large/giant-cuttlefish-24M1611-74D.jpg)
I realize that most cuttlefish are rather small - the biggest subspecies grows to about 20 inches - but then again, there are still plenty of unknowns in the murky depths.