"In the meantime, we urge you all to stay calm and remember that cows don't kill people. Everything ELSE in Sanctuary kills people."(src)
Cows have appeared in various installments of the Diablo series.
An easter egg existed that if a player clicks on a cow eight times, their character will say: "Yup, that's a cow all right!" Clicking a few more times will have them say, "I am not thirsty" and "Hey! I am no milkmaid!" The messages will cycle through those lines again if you were to continue clicking on the cows.
It was these cows that sparked the rumor of the Secret Cow Level. The story went that by clicking on a specific cow in Tristram a certain number of times, one could access a wondrous place known as the Secret Cow Level.
Cows are present once again in Diablo II, first sighted from the very start of the game, standing in the Rogue Encampment. While out slaying monsters, cows may be occasionally found standing under small shelters.
These cows cannot be interacted with and neither will the presence of vicious demons, slaughter, nor magical explosions have any effect on the cows' peace of mind. They are, more or less, just decoration.
In the ruins of Tristram, the dead bodies of cows may be found, presumably they represent the cows of Diablo I that have been destroyed along with the rest of the town.
These dead cows do not indicate anything when the mouse is run over them, but they can be interacted with, unlike the living cows. Clicking on the dead cows will cause them to explode violently, dealing damage to the player and spattering entrails around. This effect is not unlike a Corpse Explosion, although triggered more like a trap rather than a cast spell.
The game also features the Secret Cow Level, with the appearance of Hell Bovines.