Hardhat Beetle

The Hardhat Beetle is a relative of the Octorok, and is sometimes also referred to as an Octogon. This enemy does not really attack in most games, instead, it acts as an obstacle. They are often immune to damage from the vast majority of Link's weapons.

A Link to the Past
In A Link to the Past, the Hardhat Beetle is only found inside the many dungeons of Hyrule. These creatures' outer body is so thick that if Link were to slash them, he would be repelled back. Your best bet is to bounce them into a hole or to blast them away with a Bomb.

Link's Awakening
In Link's Awakening, these could only be knocked into pits, eaten by Bow-Wow, or destroyed with a bomb. They could be stunned with the boomerang, however.

Oracle of Ages and Oracle of Seasons
There is a required minigame in Oracle of Ages that demands that you defeat four of them in less than thirty seconds or before the minecart hits whatever item is being fixed.

In Oracle of Seasons there is a ball that can be manipulated using the Magnetic Gloves. This ball can cause damage to a Hardhat Beetle.

Four Swords Adventures
The blue kind also appeared in Four Swords Adventures, where Link could either knock them into pits to kill them, stun them with the Boomerang and slash them several times, or use the Roc's Feather to jump over them and come down on them with his sword.

Phantom Hourglass
In Phantom Hourglass, they appeared again, this time with a new weakness. Instead of knocking them into pits, you can simply smash them with the Hammer to defeat them



Trivia

 * This enemy seems to resemble the Blooper and Buzzy Beetle enemies from the Mario franchise.