Blargg

A Blargg is an enemy in the Mario series that debuted in the game Super Mario World. Blarggs are red, dinosaur-like creatures that appear in lava-filled caverns.

Super Mario 64 Depiction
In the files of Super Mario 64, an unfinished model of Blargg can be found. It seems to supposed to take the appearance of it's Super Mario World form, judging by comparison of the sprite from SMW and the model from SM64. In Super Mario 64, it has a red head and body with grey teeth. The coding of the game shows that it was supposed to appear in three levels of Super Mario 64, usually: Lethal Lava Land, Bowser in the Fire Sea, and oddly enough, Wing Mario Over the Rainbow.

The behavior of Blargg is reported to be similar to how it behaved in Super Mario World: it will start out sitting in the lava, then rise out of it. After the rising, Blargg would move towards Mario's location. If Mario is in contact with Blargg, he will be burned the same way as when he touches lava.

It is unknown why Blargg hasn't been finished for the game. One possible theory would be like how many features of games went unused or unfinished; due to time constraints.

Anomalies
However, due to the personalization of copies of Super Mario 64, sightings of Blargg could be noticed in some copies of the game. As it is a new entity on its own, it does not seem to be caused by Dynamic Level Rearrangement. However, the placement of Blargg can be affected due to the algorithmic change. It only seems to appear in the three stages listed above commonly. Unlike some anomalies, Blargg doesn't seem to have any malicious side effects or phenomena tied to it. No side effects were reported when the enemy had appeared.