From Fallout: New Vegas, in the Old World Blues DLC.
Essentially, they are long-dead skeletons, still kept mobile and moving thanks to the Override Harness.
It was originally designed to help transport wounded soldiers back to safety, but as some unfortuate victims found out, the user wasn't required to be alive for the suit to operate...

I alway found them a creepy concept, and decided to make a skin!