The Japanese title also provides another shred of information about the beast. The Last Guardian is known as Hitokui no Oowashi Trico (loosely Trico: The Giant Maneating Eagle). I just learned Oowashi refers to Haliaeetus Pelagicus, a sea eagle specific to Asia and considered to be the heaviest eagle in the world.
I suppose the creature is based off H. Pelagicus. Do you think Team ICO’s creation looks like the eagle at all?