Commander Keen IV: Secret of the Oracle
Secret of the Oracle is the 4th episode of the Commander Keen series, developed by id Software in 1991. The whole episode has been released as shareware as part of Commander Keen in Goodbye Galaxy, a mini-series containing two episodes of the full series.
This episode is the first game where the Dopefish appeared, a large green fish with buck teeth. It has been described as "the second-dumbest creature in the universe". Since then, it already made hidden appearances in more than 30 computer games as an in-joke of the PC game industry.
This game has been mentioned in "A Visit to id Software":
• A Visit to id Software (November 1993)