I found myself copying what I saw in official texts, no matter what sense it made. So: mudokons, sligs, steef and animal species not capitalised; Ratz other species, live ammo (discluding fuzzles) all capitalised.
I do the same for items pluralised with "Z"s and "S"s.
But I agree, we do need consistency. However, capitalising "slig", to me, feels strangely obscene.
And from one who knows: when writing about these creatures the page gets incredibly messy with so many capitalised words, and they come up alot in Oddworld fiction.
My suggestion would be to capitalise them when specifically mentioning the species, in a list, description or otherwise treating them as a race. But in passing, in the main body of text where these words are simply part of the flow, keep them in lowercase. Although special care has to be taken with Interns and Outlaws, it isn't difficult to establish them as a species in exactly the same way and then continue using the word in the conventional manner. If you have any competence in writing at all, your reader will know what you are talking about.
As for s/z, I'd abolish "Z" entirely, except in names (eg "Vykkerz Labs") and specified examples, such as "ratz".
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