A type of rust monster that mimics animal calls or other sounds when it can't reach the player directly.

[Cenith]

I just had the eeriest experience in my game. 

Sitting in my dirt hut, heard a wolf howl. It sounded very close, almost right on top of me.
Got my spear, went out to investigate. 
As soon as I open the door I was pounced on by a shiver.
Took my completely by surprise as it was daylight, rift activity was calm, and I hadn't heard any other sounds. 
Killed it, looked around. 
No wolf. 

It made me feel like the shiver had mimicked a wolf howl in an attempt to lure me out of my hut where it could get me

And that sounded like a neat idea to share - a rust monster that, if it can't path directly to you, attempts to lure you out by mimicking either a random or nearby animal. Maybe even a trader voice, or the voice of the player character. If you have livestock, it could mimic their distress/hurt sounds. If you have a resonator, it could copy the last cylinder you played. All kinds of options!

[Diff]

If a Bell was ever to wander all the way up to the surface for some reason it'd make an excellent candidate. Or maybe just a bell head.

[Bruno Willis]

42 minutes ago, Cenith said:

And that sounded like a neat idea to share - a rust monster that, if it can't path directly to you, attempts to lure you out by mimicking either a random or nearby animal. Maybe even a trader voice, or the voice of the player character. If you have livestock, it could mimic their distress/hurt sounds. If you have a resonator, it could copy the last cylinder you played. All kinds of options!

All of the above! I love this idea. Sounds like the sort of rust foe that might follow you up out of a dungeon and get you when you least expect it.