A mob which is skulled, is a monster which is hostile towards you, 10 or more levels above you. A small skull icon (
) appears in place of the level circle on the selected portrait. Skulled mobs are also often referred to as level ??.
If you hover your mouse over them, they will appear as a level ??.
e.g. A level 30 orc sees a level ?? Night Elf Hunter. The hunter in question, could be of any level ranging from 40 to 70.
Once level capped, if a mob still appears as a ?? then they are referred to as a "boss" mob. This is done by Blizzard to hide the mob level.