I guess the thing Bungie is getting at is that any sufficiently advanced intelligent species would probably resemble what we call a humanoid form. Think of it as universal convergent evolution, any species which utilizes intelligence as an evolutionary trait would evolve in such a way as to free up it's hands so as to better manipulate the world around itself increasing the chance for the species to prosper, grow and reproduce. I'm sure the Fallen have probably domesticated certain other species they have discovered during their journey through the universe and perhaps we may see them. Now they (The Fallen) haven't advanced so far as to exist in a purely ethereal form which interacts with the universe purely through will, but perhaps that's what the Traveller is, perhaps it's the key to the next stage in intelligent life. The Traveller reminds me of the Monolith from 2001 a space odyssey. I reckon it predates all species in the universe by 100s of millions of years. So perhaps the species who created it have gone beyond a solely humanoid form.