: So, the first question is, what, in total, do we know about these other
: species?
You've summed up everything we know about these species, but I'm sure you knew that already.
: The second question is, how do you imagine these species in your own head?
There's an old thread asking a similar question, and Godot replies to that thread with a link to an even older thread asking a similar question.
As for answering your question about how do I personally imagine these species that we have literally no description of whatsoever...
I picture the Nakh as cybernetic starfish the size of a person; it fits with the Jjaro uplifting races like the S'pht and starfish-people seems outlandish enough to be used by Bungie as an early species of the universe. I picture the Vylae as a faceless, gaseous lifeform wearing cloaks and robes. I picture the Nebulons as the ripples from a pebble thrown into a still pond in the morning. I picture the Nar as a single thorn shaped like a claw hanging from the branch of a tree. I picture the S'pht as winged, beaked beasts who screech and crawl on their bellies across the yellow brings of their jungle temples, pressed to the Earth by the light of the sun. I picture the Pfhor as a mummified corpse, standing upright in a room of darkness staring up at the beam of light from some unknown spot in the ceiling. I picture the humans as a stain on a mirror which dances around its double as you shift your gaze. I picture the 10th Mjolnir Cyborg as a body of fire with the face of an ox, the face of an eagle, the face of a man, and the face of a lion with horns.
Hope that helps. Go finish Marathon: Grendel.
