Well my origional view of a Calliach too, was essentially a 14-foot tall Myrkridia with a magic staff :)
We know though that the watcher is non-human (he seems to be at LEAST 1.5x the size of my zerks!) And we have not seen under his mask OFFICIALLY in the game. All we've seen of his skin is his right arm while hes turned to stone. As far as we know, he could have a tail with a spiked ball on the end and we would not know.
And also, there is much more to humanoid that physical body shape. Perhaps the Calliach were so vicious that they were inhuman, they were so cruel and bestial that they were not human in any way.