Yeah I threw out borderlands and fallout just cause most people don't think of them as FPS so i thought it's possible you haven't run into them.
The issue is once you remove scifi/creatures from an FPS, all you're really left with is humans, resulting in everything being mostly military/war.
One other game I thought of that's an FPS but a bit out there is Neon White
If you're willing to look at older games then it does open up a bit too.
The Wolfenstein series isn't indie, but meets your asks. It's an alt history FPS based in WWII, so most creaturish you'll get is dogs.
Outside of that fallout and borderlands weren't mentioned yet, but both have rpg elements and creatures. Fallout is alt history USA while borderlands takes place on another planet.
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