Oh I dread the day we get procedural flora and fauna in ED. It's what I really hated when playing NMS, not just the overabundance, but also, with all the procedural diversity, it all looked... the same. I really hated that. And if we ever do get fauna, it should be the exception, not the norm. ED strives for a certain level of realism, and the real galaxy is just... empty for the most part. Probably.
Depends how much constraint you apply.
I mean, you can start off by being selective about the planets where fauna spawns.
After that, you look at what else is there. If a planet already has
something that could plausibly be considered a nutrient then it might be able to sustain life.
Then you'd look at how
much "food" there is on a planet, and how it's distributed, to decide where, and how many, creepy-crawlies to spawn.
As for the beasties, themselves, "realism" could easily be maintained by creating a library of suitably realistic body parts - legs, heads, eyes, abdomen, thorax, mandibles, antenna, greebles etc - and then scaling/morphing them, mixing and matching them and colouring them all within set limits to generate unique but similar-ish lifeforms.... which, again, would be plausible insofar as it's likely similar environments would evolve similar creatures.
Also, we're not talking about the sort of goat-elephants that appear in NMS, here.
I'm thinking more about things like flatworms, lamprey, crabs, insects, spiders and tardigrades.