Blargg

Blarggs are unused enemies in Super Mario 64. They are referenced to appear in Lethal Lava Land, Bowser in the Fire Sea, and oddly enough, Wing Mario Over the Rainbow. The third stage is believed to have been a lava level due to the Blargg being referenced to appear there in the files. There are 2 animations for the Blargg of it jumping out of wherever they are placed, usually lava, and hiding inside of whatever it is hiding in. The model for the Blargg does not have any textures, and does not appear to have eyes.

Finished Appearance
In some personalized copies of Super Mario 64, a finished version of the Blargg can appear in any of the 3 stages. In Lethal Lava Land and Bowser in the Fire Sea, there will be a new grate platform object that the Blarggs can jump through. In Wing Mario Over the Rainbow, the Blarggs will jump out of the clouds.

Boss Blargg
In copies that have finished Blargg enemies, there is a chance to encounter a Boss Blargg, a larger version of the Blargg that functions as a mini-boss. It will usually replace a Big Bully in Lethal Lava Land, and the platform whichever Big Bully it replaces will be replaced with the aforementioned grate platform. It is usually defeated by ground pounding it's head 3 times. It can also appear in Boiling Hot Basalt, and functions the same.

Blue Blarggs
In some copies of Super Mario 64 that have blue lava in Lethal Lava Land, there is a chance of a Blue Blargg appearing. These Blarggs usually gain the ability to shoot fireballs at the player. The Boss Blargg variant also usually gains the fireball ability, but can also gain arms that can be used to attack the player.