Is there already a monster like this?

[Yqatuba]

Is there a monster that's like a living suit of armor with nobody inside?

[Pizza Lord]

Aside from an animated suit of armor, the Helmed Horror is the most common variation of animated armor. I don't think it's official Pathfinder, however, though a conversion probably isn't hard to find.

[Derklord]

The Phantom Armor from Carrion Crown book 5 is what you're looking for. The Giant and Guardian veriety are reprinted in Bestiary 4.

[Yqatuba]

Awesome. I like the idea of a villain wearing one and the players wouldn't realize it's actually a monster until the villain dies and the armor comes off and starts fighting on its own (does this happen in the book? I've never played it)

[deuxhero]

You may want to look at Graveknight if that's what you want to do. That's exactly what it was designed for.