The Secret Cow Level in Diablo II — a pasture of axe-wielding Hell Bovines with their own boss, the Cow King — began as a rumour from the original Diablo. Players reported that clicking a specific cow in Tristram enough times opened a secret level. Blizzard confirmed there was no such level. The secret never existed.
For Diablo II, Blizzard added the actual Cow Level as a direct response to the rumour — a reward for players who had spread the myth for three years. Reaching it requires combining Wirt’s Leg with a Tome of Town Portal in the Horadric Cube after completing the game, creating a portal to a level whose existence was never officially confirmed until players found it.
The Cow King boss — who, if killed, prevents that character from reopening the Cow Level portal in that difficulty — adds a mechanical consequence that the original rumour never imagined. Blizzard built friction into the fantasy they were delivering.
The Secret Cow Level is the most direct example of a developer turning a community-created myth into a real easter egg. The community invented the story; Blizzard made it true and then made it cost something.
The detail work the devs put into areas most players never visit is what separates great games from good ones.
I’ve put 200 hours into this game and never caught this. The developers really reward the obsessive players.