Well, the _1stperson folder is brimming with animation files. Have you installed any animation mods lately?
Nope, unless it came bundled with one of the mods I installed. I'll look through them later today. Right now this is the only thing in the _1stPerson folder
dynamicidle_sit
Same with creatures. Any creature mods get installed lately?
Yep. MMM, and I'm quite sure many of my mods have modded creatures in them. Again, I'll look around later.
These are the three files in the skeleton folder.
BHelmet
helmetlich
SKDBCuirass
EDIT: Did a quick browsing around. Evidently, the helmetlich file had its origins in The Dark Tower. (A mod that I had initially installed, but upon rechecking the tesnexus page, found that it conflicted with heart of the dead. It was deactivated before I began the OP.