So, let's put this in concrete terms. Rats are dangerous too, in giant hordes. But so is any weak creature. Here on the Codex, we call them trash mobs.
As for games. Off the top of my head, rt group with rpg elements, with a few rt no pause co-ops in there:
- Trapped Dead
- Trapped Dead 2
- Survivor Squad
- Breach and Clear: Deadline
- How to Survive
- Dead Island: Epidemic
- Guns 'n Zombies
- Dead Horde
- Nation Red
But let's go beyond that, to the horde of hordiest horde games - Dead Rising. In there, you can be absolutely surrounded by zombies, alone. And is there any challenge even then? Fuck no. You can get a kill count of over a thousand in less than 2 minutes. Zombies are barely even a nuisance soon as you get a baseball bat, because a bat puts you leagues ahead of the fighting ability of even a zombie horde.
Because what is a zombie? A brain-dead moron with no tool-use, no natural weapons, no organization, no survival instinct. That means that, tactically, they are lower than Stone Age opponents. Think about that a moment. Now realize that a couple of gatling guns have mowed down 10,000 equipped, reasoning opponents in a single combat. Now, just picture what happens when you take away the reasoning from the enemy and give their opponents aerial bombardment. Or hell, do what they did in the Old West, get a bunch of townsfolk on the rooftops, and gun the bastards down. You're shooting down enemies with less survival skills than your average fish, after all, and townsfolk would get rid of large bandit gangs that way.
Yet, let us go even beyond that, to the essence of what is a game and what is horror. The main ingredient of horror is the unstoppable force. The fact that one cannot fight that force is what makes it frightening. Soon as you make it concrete, soon as you make it killable, there is no more fear. Which is why horror movies save the monster for the end. Or, in the keeping of survival horror, make the horde itself unstoppable. Anyone who stands and fights gets a few kills, but soon dies. Only by running do you survive. And so, you run and you run and you run, until you can't run no more, and then you die too. That's the horror. Which is a very concept the runs entirely counter to what RPGs and action games are all about. Soon as you take the horde and make them killable, you lose the fear. And then all you're left with is a creature dumber than a fish with no fighting ability, and lots and lots and lots of them that the player has to wade through to get to the end. The trashiest of trash mobs.