The original "Diablo" game, released by Blizzard Entertainment in 1996 for PC and Mac, had players traversing the village of Tristram to fight various evil creatures, demons and powerful boss characters. But not all the fauna of Tristram would try to kill you. There were also harmless cows, and their existence was apparently enough to spur a rumor that you could enter a secret cow level by clicking on a particular cow a bunch of times.
It wasn't true. No number of cow whacks would cause a portal to a bovine-filled realm to open. But the rumor had such traction, and Blizzard got so many questions about it, that it was referenced in later official Blizzard games and eventually became a real thing.
The game "StarCraft" included a cheat code, "There is no cow level," that would apparently cause you to get instant victory on the current level. And in "Diablo II," a real secret cow level can apparently be reached by completing the Eve of Destruction quest on one difficulty setting, traveling back to the Rogue Encampment and using the Horadric Cube to transmute the Tome of Town Portal and Wirt's Leg. After all that work, a red cow portal will appear to take you to a level not full of the docile creatures you would expect, but to one full of armed and dangerous Hell Bovines and a boss character called the Cow King who throws lightning bolts. Also, in Blizzard's MMO (massively multiplayer online) RPG game "World of Warcraft," you can obtain an item called the Cow King's Hide.
Nods to the secret cow level aren't confined to Blizzard. The MMO expansion of "Goat Simulator," a game by Coffee Stain Studios that is exactly what it sounds like it is, includes the town of Twistram, where you can find a passage to a level full of killer cows — although these shoot laser beams from their eyes.