To celebrate the third anniversary of Diablo III, an in-game event is going on now until the 21st! Cows can be found throughout Sanctuary. Upon their death, they will open a portal to a secret realm filled with cows, dead farmers, and plenty of treasure! Farmer corpses give lots of gold when activated.

Check out the news link below:
http://us.battle.net/d3/en/blog/19083832?utm_content=external-sso&utm_medium=other&utm_source=App