This happens alot on random mobs actually...
It's just when a mob is spawned and it takes something to use for it self. Like the bear riders in the Storm peaks, the daily quest, Back to the pit.
Try attacking a bear with no-one on it. You will get this error message.
Chillmaw, the dragon for the daily quests has it aswell. When killed, it will respawn rather fast, but it will need to bring the gunnaneers on itself, and sometimes that goes a little slow.
The error is just to prevent that players won't be able to attack a mob / Npc that is not ready for it.