As we can read at Metal Gear Wikia, Guy Savage is the name of an easter-egg mini game that can be found in Metal Gear Solid 3. This mini-game was directed by Shuyo Murata (MGS3 writer) as stated in the credits. Guy Savage is a basic hack-n-slash game where the player encounters a vast amount of enemies, and must press the attack buttons repeatedly to defeat them.
While there’s not an official statement, rumors say that Guy Savage was supposed to be a Konami game in development, that was later cancelled. The playable demo that was created as an internal pitch for the project was added in MGS3 just for fun.
Thanks to Userdante for the contribution!