Freeing Marty the Thwomp in Mario Kart 64 isn't a standalone game but rather a popular glitch or trick within the classic kart racing game. Players discovered ways to manipulate the game's code to 'free' Marty, a normally non-playable character that typically acts as an obstacle in the game.  The process itself varies, requiring specific actions and timing to cause this in-game event. It wasn't created by a developer, but rather discovered and spread by the player community, becoming a notable example of player experimentation within older video games.   The original Mario Kart 64, developed and published by Nintendo, remains a beloved kart racing title with a huge legacy.

Freeing Marty the Thwomp in Mario Kart 64

Forsaken 64 (Germany)