Pony Kart 64 is a fictional game. There is no known official game with this title released for the Nintendo 64 or any other platform. It is likely a fan-created concept, or a name similar to other kart racing games on the system.  There's no gameplay, developer, or history to discuss as it's not a real game. The significance is purely hypothetical within fan discussions or creative writing about games.

Pony Kart 64

Playable Toad in Mario Party 3