While defending my fort, I've seen a skull icon appear over an attacking Husk, but haven't been able to figure out what it means. It's happened a few times now, but I was finally able to get a screenshot in time last night.
What does this skull mean? If it means anything, I've only ever seen one of these icons at a time and it's always been while defending forts, though I don't believe it's necessarily always been on Husks that were actively attacking a structure.
Best Answer
Although this Reddit seems to be discussing a different kind of skull, someone also pointed out that the skull above enemies simply means that they have been marked:
This Reddit with a title of "PSA- You can "mark" enemies for other players to see, it will add a skull icon and is easily noticeable by everyone." also seems to confirm (they did it on Xbox One):