Hardhat Beetle

Hardhat Beetles are enemies that resemble Octoroks. They are often immune to all of Link's attacks, and they must be pushed off a cliff in order to be defeated.

The Legend of Zelda: A Link to the Past
In A Link to the Past, the Hardhat Beetle is 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.

The Legend of Zelda: Link's Awakening
In Link's Awakening, they can be defeated by being knocked into pits, being eaten by Bow-Wow, or being destroyed with a bomb. They can be stunned with the boomerang, however.

The Legend of Zelda: Oracle of Ages and The Legend of Zelda: Oracle of Seasons
There is a required minigame in Oracle of Ages that demands that you defeat four or eight 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.

The Legend of Zelda: Four Swords Adventures
They appear in Four Swords Adventures as well. Link can 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.

The Legend of Zelda: 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.