Sega Master System / Mark III / Game Gear
For a chance to challenge the alien champions and become the wrestling "Master of Universe"; at the game over screen after becoming World Champ, press any direction, 25 times.
At the title screen, press on controller 2. A secret message (in Japanese) will appear:
Translation: "Hey, it's true!"
See the main page for Pro Wrestling / Gokuaku Doumei Dump Matsumoto (極悪同盟 ダンプ松本)