Nargott
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There's no need to invent anything; there are classic creatures/swamp creatures that wouldn't raise any questions. And not beast-like, but specifically reptiles/snake-like. Otherwise, it would be the same mishmash. New races? Then you could try looking at Age of Wonders 3; borrowing from it would at least guarantee that the faction is a cohesive faction, and not a mishmash with human units. Tigrans, Frostlings, Draconians.