Pomni in SM64 refers to a fan-made hack or mod of Super Mario 64 for the Nintendo 64, featuring Pomni as a playable character. Pomni, an original character, was created by the modding community and introduced into the game via custom modifications to the game's code and assets. Gameplay involves navigating the existing SM64 levels with Pomni's unique abilities (if any are specified in the mod).  Not being an official Nintendo release, it was created by independent developers within the modding community and has no official publisher or development team.  Its significance lies in demonstrating the creativity and ingenuity of the modding community and the longevity of SM64's appeal.

Pomni in SM64

Playable Bowser in Mario Party 3