This month only there is a special cow level accessible from a portal in the Immortal King's throne room in the Ruins of Sescheron in Act 3. A link to the article is pasted below.
http://kotaku.com/diablo-iii-players-discover-new-secret-cow-level-1762347713-420
EDIT: The Immortal Throne is located in a side area of the ruins called the "Elder Sanctum". Just tried out the cow level, it's relatively small but the cows are densely packed.
