Generally speaking, a dev thread for species is broadly similar to what you'd write out for a piece of tech. You're basically establishing any abilities they might have, unique features, so on and so forth. How you go about it is up to you, but a lot of folks like to take the role of the researcher discovering them for the first time.
As far as the whole not existing thing goes, the Codex is completely optional. You can write the species to your heart's content without having it approved. Approval grants it inclusion into Chaos canon, however. This basically means you can write the species in open RP events like invasions and the like and no one can complain, so long as you don't abuse it.
Hope this helps.
[member="Bat"]