its not cthulhu, according to jj himself this is an original monster. one we have never seen before...
for some reason i'm leaning towards some a squid or whale like monster. possible a mixture of both, one of the titles the film has been in production under is "the parasite" so it very well could have binded itself to a large squid then maybe to a big ass whale or vice versa...who knows, whatever this thing may be it's large, apparently twice the size of the statue of liberty it has some kind of claws, it has a monstrous roar and its mean as hell and comes from the ocean.