Diablo 2 featured a huge Easter egg in the form of the secret cow level, where hordes (or rather herds) of Hell Bovines converge on the player. So we're sure players all around the world have been trying to look for a similar Easter egg.
And according to a report from the Verge it seems four Diablo 3 players have succeeded. While the secret level dubbed Whimsyshire in Diablo 3 doesn't contain any bipedal homicidal cows, it does however house unicorns. Probably in reference to the backlash faced by the game's designers regarding Diablo 3's more colorful art style, the hidden level contains a bright and colorful palette of colors with rainbows and unicorns. If you don't mind any plot spoilers and want to find out how to get to the secret level, hit the source link.
Source: The Verge