Blargg is a character from the Super Mario series.
While Blargg was meant to be reused to be an enemy, just like in the earlier installments of the series, he never made it out of the beta stage of Super Mario 64 and thus ended up as a cut character.
The Blargg found the files of Super Mario 64 doesn't appear naturally, but can be hacked into the game or made to appear via cheat codes. He was meant to appear in Lethal Lava Land.
Unlike his counterpart from Super Mario World, Blargg from Super Mario 64 uses a 3D model, that has been fully modelled but lacks a texture. If Blargg is forced to appear ingame, he will have a red body and grey teeth.
Forcing Blargg to appear ingame, will make him behave similar to his Super Mario World counterpart. He will start out sitting in the lava and will sink and then rise from it, if Mario approaches. Afterwards, Blargg moves towards Mario and will burn him, if he makes contact with the fiery creature.