Buzzy Beetle

Buzzy Beetles are a common enemy to be seen in the Super Mario Bros. series, acting as Koopa-like entities that cannot be destroyed with fire. They're suspiciously missing from Super Mario 64, though Buzzy Beetles might show up in certain personalized copies of the game.

Super Mario 64 Manifestations
Buzzy Beetles can show up in some personalized copies of Super Mario 64, often replacing the Money Bags in Snowman's Land or Scuttlebugs in Hazy Maze Cave. Their behaviors would vary, but most of the times they'll walk around freely, damaging the player by 1 upon contact. When they're defeated, they might also drop an object known as a Hard Shell. They are also reported to appear in the Cavern of the Metal Cap.

Drops
Upon defeat, most copies of the game would let the enemy drop a Hard Shell. The functions of this power-up would be different between copies, but most would feature it as a throwable or kickable weapon that can damage enemies and destroy bricks, similar to a wooden box. Other copies would feature a Hard Shell as a ridable tool, almost identical to a Koopa shell. But Buzzy Beetle shells would be able to bounce back when bumping into a wall, and it could be used to transport through quicksand, even though it'll never show up in Shifting Sand Land.

Theorizing
Some believed that the Buzzy Beetle was once planned to appear in Hazy Maze Cave, but was later scrapped because of its similarity to Koopas and boxes. Its appearance in Snowman's Land can be explained by its almost identical appearance compared to a Moneybag. Though the developers didn't put them in the levels directly, but the Personalization AI can actually pull it from the game cartridge.