Sometimes you just want to give your characters something to fight and survive. No pesky moral questions, no politics, no negotiations; just kill or be killed. At the end you’re alive, someone or something else is dead, and you can just keel over for a while. To the modern reader who’s often stressed beyond bearing but has no good outlet to go take out all the rage and primal screams on, characters surviving can be very cathartic.
If that’s the fight you want in your story, then you probably want a horde. Individual threats are, well, individual. They tend to lead to character development, a la Dracula from generic undead menace, or even just the anthropomorphizing of a particularly vicious storm or cliff face. (If the characters don’t give such a threat hints of emotion and motive, the readers will. Humans are human that way.)
Hordes, though – hordes by their nature are a mass of threats, all blending together. A swarm of piranhas, a circling school of sharks, a stench of zombies. Everything comes at you at once, to overwhelm the senses and rational mind. If your body goes down into freeze mode with your mind, you’re dead.
This is another bit that differentiates hordes as a threat from individual predators or villains. “Freeze” is a perfectly legit survival strategy against most individual enemies; if they don’t see you, they might miss you, and go after something else moving. Which sucks for the other guy, but….
You can’t beat a horde that way, any more than freezing works against a march of army ants. The column spreads out so it hunts farther and more thoroughly than any one predator can. Freezing just means they find you faster. It’s run, or fight. Either one gets the reader’s adrenaline out, and usually you can have a mix of both, especially if you pull a scenario like “run from the zombies across a bridge, then blow the bridge at the very last minute”.
(At which point we all hope zombies don’t swim. Heh.)
So if your story needs a horde, where do you get one?
If it’s something like an insect swarm, it’s easy to explain. Either the horde normally tramps across the landscape munching away, or it does so under certain circumstances your main characters have to fall afoul of. Bees out to form a new colony are one example, army ants another, locusts yet a third. If it’s a horde of orcs or other monsters, you have some worldbuilding to do to explain how they built up enough numbers to switch to large-scale looting, burning, etc. And how they did it before humans wised up and nipped a potential horde in the bud, because we tend to do that when we get wind of man-eating threats. Demons or monsters from another dimension are a bit easier; “they were always there, they just couldn’t get over to this world to eat people.”
This may be why zombie hordes are often popular. All you have to do is worldbuild in your Patient Zero, and things explode outward from there.
(“All”. Heh. There’s a lot of worldbuilding that goes into whether or not you want this to be a zombie apocalypse or a beatable zombie outbreak. From what I’ve read Kingdom (2019 Korean historical drama, currently on Netflix) really pulls it off for a horror that can be beaten. I’m hoping at some point they’ll put out official DVDs so I can check it out – the TV Tropes page is interesting.)
And then of course there’s the option of a horde of robots. Which might act like any of the above critters, or possibly all of them. Imagine Skynet with our modern-day drones available. Brr.
When all’s said and done, shape your worldbuilding and your heroes so they’re smart and lucky enough to survive the horde… barely. Stretch your creativity; make it more than just being fast and strong enough to fight on. One of the integral parts of being human is tool use, and the environment around us is a host of potential tools. From Improvised Weapons to Combat Parkour to Badass Bookworms remembering one obscure fact about the enemy; hit all the facets of what it means to be human, the better to contrast against the inhuman hordes trying to kill them all. Your readers will love it.
Once they catch their breath, at least….
If you don’t mind spoilers for Kingdom – have a Last Stand against a zombie horde that works!
(At this point they’ve discovered the zombie-making effect is a parasite, and it hates water, so while full zombies can and do drown, if you get someone just bitten submerged fast enough….)