A mob which is skulled, (aka "Lvl ??" or "Lvl Skull") is a monster which is hostile towards you and 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 ??".
Once level capped, if a mob still appears to the player as a "??" then they are referred to as a "skull type boss". These bosses have a "dynamic" level — they are considered to be 3 levels above the player character for any level-based calculations during combat (such as miss chances, resistances, etc).
Party members' pets may appear skulled if they are too far away for the UI to retrieve info about their level. Party members themselves will sometimes appear skulled if they join while in-flight, teleporting or in the Deeprun Tram.