Herald to the Queen

"Is that a cow Treasure Goblin? He was really hoofing it."

Herald to the Queen, Hoofin' It was a unique Infernal Bovine that could be randomly found in any zone in Diablo III that can spawn Treasure Goblins. It only spawned during the anniversary event in May 2015, and had a very small chance to appear in place of any normal Goblin (but not in Nephalem Rifts), both in Campaign and Adventure Mode.

Killing the Herald would cause a portal to Not The Cow Level to appear. Once the portal was opened, the Herald couldn't appear again in the same game.

In combat, Herald to the Queen behaved just like any Goblin, and even dropped gold as it ran, but moved considerably faster, had more Life, and did not gain a moment of invulnerability when first hit. The cow could escape if not killed quickly, so killing it as soon as possible was advised. In return, it did not have a time limit for killing. Upon death, a usual portion of Goblin loot would be dropped.

As of patch 2.3.0, one no longer requires killing Herald to open the portal to Not the Cow Level. Instead, using Bovine Bardiche in Kanai's Cube will unlock entry to the secret level.