The idea sounds interesting, but it needs to be fleshed out more.

People fear danger, and it is more of an instinct that we feel nervous and uneasy if we know that there's something there, but it is hidden from our view (the predator stalking us for his next meal). Horror games/movies often utilize this. One concept is to blend dangers into the environment. First by making an hard to overlook environment with many possiblities to hide (a destroyed urban setting, a steam filled tunnel on a spaceship, a dark house, a forstest in the night, a maze with many closed doors). Then the stalker needs to fade into the environment.

The mannequins concept is often used in this way: many mannequins are the environment, human like puppets, where the stalker, a human himself, has some way to just stand there close to his prey without being recognized easily.

Therefor, once you know that the mannequins are the predators, you can pick them out easily. It is just to easy for the player to overlook the danger of a given scene. If only one of all the mannequins is the predator, then it would change....

@Legendre: It doesn't seem like it; the concept of "ordinarily inanimate objects moving when one isn't looking, and being murderous" isn't original to Five Nights at Freddy's (and indeed, mannequins aren't entirely uncommon for this purpose, I believe). Additionally, while I don't know what sort of gameplay is intended, I don't see indications that it's likely to be built around static cameras and resource management, as I believe is the case in Five Nights at Freddy's.

ordinarily inanimate objects moving when one isn't looking, and being murderous

Makes me think of the weeping angels from Dr Who *shudder*...

ordinarily inanimate objects moving when one isn't looking, and being murderous

Makes me think of the weeping angels from Dr Who *shudder*...

That was indeed one example that I had in mind, I believe (although I've never seen the actual episode myself, I'm afraid). I've also seen actual mannequins (or similar) in a few horror games of late, although the names of those games escape me offhand, and the idea of mannequins as objects of horror is far from new (see: Silent Hill).

So perhaps its only one mannequin?

Either that, or something that can possess/animate any mannequin... but only one mannequin at a time. The key is to prevent your player from getting comfortable that they can identify or predict the behaviour of their enemy reliably.

I'd suggest a read through "Taking Slender apart, an introduction to Horror Games" -- it's looking at a specific game and not everything will necessarily be applicable, but I think you'll find some good general tips for potentially making a very effective horror game.

A thought: another source of fear might be the experience of being watched, especially by something that shouldn't be able to watch you--like a mannequin. Perhaps it might be worth having the mannequin at times turn up in a position from which it isn't in a position to attack the player (such as behind a shutter or glass storefront, or behind bars), but from which it simply, silently watches the player. Think of Silent Hill 2 and the early sighting of Pyramid Head behind a set of bars in a corridor, for example.

Darkness and creepy noises..

And don't forget NOT ALL of the mannequins are the evil ones (some are JUST inert mannequins). Keeps the player guessing especially when the evil mannequins are sneaky enough to reposition the non-evil ones.

If this was a 3D graphics game then the many strange poses of ALL the mannequins might be an amusing aspect (so you can laugh just before you DIE!!)