The Rogue Bull, sits in a pond in Lakeside County waiting for people to get too close so that he can charge at them. The bull has been doing this for a while now and has become quite a nuisance. So much so, that Lady Althea wants someone to "take care" of him in the quest A Mesmer's Burden. Bill, as it turns out, is owned by Old Mac, who would be quite upset if anything were to happen to him.
The Rogue Bull has relatively high armor. Killing him with a weapon might take some time; it's dangerous if your character is level 1. The best way to damage it is through health degeneration (e.g. Conjure Phantasm, which Althea offers in the quest) or armor-ignoring attacks as in Smiting Prayers. The Rogue Bull has 80 total health.
The Rogue Bull is not a boss, though it is one of the few creatures in Pre-Searing that has an elite skill.
Though often mistaken for an animal by inexperienced players in Pre-Searing, the Rogue Bull is not tameable as a pet.
The Rogue Bull respawns after visiting a town or outpost, so you can kill it as many times as you like until you complete the quest. It's level five and therefore good for lower-level characters to gain early XP from.
The Rogue Bull can be knocked down, though it appears to remain standing.