I've had the bullete appear in both the minotaur and hook horror fights. It sure seems to get around.
But what I've found interesting is that it can turn other Underdark denizens hostile, I assume by catching them with its aoe. In the minotaur fight one of the minotaurs went from attacking my party to attacking the bulette (and killed his companion in the process). The same happened in the hook horror fight; they went after the bulette once it appeared. They actually helped my party defeat it.
