Slime monsters as villains always felt like a weird default to me, but the more I read, the more I see the clever utility of them, especially in stories with RPG mechanics. They're the perfect early-game obstacle, a tutorial boss that teaches the rules. In 'That Time I Got Reincarnated as a Slime', it flips the script entirely, but in most classic fantasy or litRPG, a slime is the first thing you encounter outside the village gates. It establishes the power system—maybe it's weak to fire, or it splits, showing the reader how combat and magic work. It's a low-stakes way to introduce the world's logic without overwhelming the audience.
Beyond that, I think there's an underestimated horror aspect. A slime isn't just a monster; it's an environmental hazard, a creeping, corrosive force. It can dissolve gear, seep through cracks, and absorb its victims. That shift from a simple blob to a suffocating, inescapable threat is terrifying. It plays on primal fears of being consumed or digested, of losing your physical form entirely. A sword does nothing against a thing with no bones to break.
Finally, they're narratively flexible. A slime can be a mindless dungeon cleaner, a mutated alchemical accident, or the degraded soul of a forgotten hero. That blank slate lets authors project almost any backstory or magical property onto it. Their popularity isn't just about nostalgia for old video games; it's because they serve so many foundational storytelling purposes while being fundamentally unsettling in a way a goblin or a wolf just isn't.