I've noticed apocalypse monsters usually come in two flavors: a physical, overwhelming threat that forces characters to adapt or die, and a psychological one that breaks down what's left of society from the inside. Take 'The Road'—sure, no literal monsters, but the cannibals serve the same narrative purpose, pushing that absolute boundary between 'us' and 'them.' The real meat of these stories isn't the gore, but watching how people organize, or fail to, when the old rules are gone. Monsters just make that process more urgent and visually dramatic. They're the ultimate test of whether cooperation or pure selfishness is the better survival strategy. I'm always more interested in the factions that form in response to the threat than the monster fights themselves. That's where you see the real human condition, stripped bare.
Some monsters are basically walking metaphors, too. Zombies often represent mindless consumption, or the fear of losing individuality in a crowd. It's not subtle, but it works. Lately, I've been bored by stories where the monsters are just mindless killing machines, though. Give me something like the creatures in 'Annihilation'—weird, incomprehensible, changing the environment itself—that's where the horror feels fresh and the survival stakes get genuinely unpredictable.