This post is for all of you who have never played previous versions of Diablo. Everyone who has been playing as long as me, will know what this is about...
Yep, that's a cow alright
The legend started with the cows in Diablo (1996). The first version of the game did not have a cow level. Instead there were three cows, standing in a corner of town. When clicked often enough, the player's character would say the legendary words "Yep, that's a cow alright".
Before long the community started all kinds of theories about a secret cow level. There were elaborate articles that described how to get there. None worked, but they were fun to read.
The Moo Moo farm
In Diablo II (2000), a real cow level was introduced. After finishing the game, a portal could be opened to the Moo Moo farm. A secret level containing vast herds bardische-bearing bovines.
If you were lucky, a set item of The Cow King's Leathers dropped, after killing the Cow King.
Not The Cow Level
Although Blizzard denies the existence of murderous cows, a cow-level does exist. Just so they can keep denying it's existence (even today), it is called "Not The Cow Level". It is the place where you meet the Cow Queen and the ghost of the late Cow King.
You may enter this area by killing a Herald to the Queen (a treasure-goblin-like cow). Once killed, a portal will open that takes you to a secret level where you can start butchering herds of murderous cows.