@IAmInept
@Belfegrad
A correct translation would be to simply state "Demons" or "Demon kin", "Demonkind" doesn't exist since demons are not a species or a race of any kind, but rather the antithesis of "good", the incarnation of "evil". On a similar and related note (seeing as most isekai do this), "Humankind" is not exactly correct, since we already have the right word: "mankind". There's also the other similar word, "Humanity", for those who want to try and be more "neutral" (hint: it still includes man in the word itself). And whoever thinks "man" within those words refers to "man" as in the male part of the species, you're plain wrong, even if both words stem from the same origin.
As far as the Korean translation goes though, since Korean is a type of language that can combine terms without modifying the meaning of the original word, and English doesn't (even if it's far more versatile than most other western languages), the correct translation for that sentence would have been "Besides the horse Esil rode, this is my first time seeing [an enemy/foe]/them ride something other than a demon/demonic/diabolic steed".
I'm also very confused as to why most authors seem to want to call the monsters or whatever they're fight against "enemy", or maybe it's the translators? I doubt it though. Can't they just say "monsters" or "insertspeciesname"? It looks really stupid honestly, no one speaks this way even during a war, and I say this knowing this is comfort literature.