Mortal Kombat arcade Reptile secret boss fight
Mortal Kombat was the first versus-fighting game to feature a secret character called Reptile, the idea was so popular that it has spread to many other fighting games.
You could only face him in one special way: if there are silhouettes gliding past the moon at the Pit stage (this will occur every 6th game) player had to achieve a double flawless victory, never press block, and execute a fatality to face off against the mysterious green ninja.
The popularity of this mysterious character allowed Reptile to return in Mortal Kombat II with his own moveset and fatalities.