Who is the first boss in Diablo 1 called?

Because of the way the quests work (sorted into five groups, with a set number picked randomly from each), and the optional nature of most quests even if you get them, you might not even fight a 'boss' encounter until you meet Archbishop Lazarus.

However the first 'possible' boss is called The Butcher, and you have a 2/3 chance of being given the quest to kill him. The next possible boss is King Leoric, aka The Skeleton King, who you have a 50% chance of encountering.