From Guild Wars Wiki
The Rogue Bull is an animal that requires defeating as part of the quest A Mesmer's Burden. His pet name is Bill. He belonged to Old Mac. He will respawn until you kill him with the quest active, after which he will not return.
- Ascalon (pre-Searing)
Quests involved in:
During Annihilator 2: Searing Day
- Unlike other level 5 creatures, the Rogue Bull has 80 health. However, it has high resistances against all kinds of direct damage. This appears to be encourage the use of Conjure Phantasm during A Mesmer's Burden.