Hardhat Beetle

The Hardhat Beetle is a jellyfish-like monster that first appeared in A Link to the Past and is notable for it's extreme resilience and elasticity. They consist of a round, bouncy shell-like bell atop a mass of tentacles that it uses to creep about the various rooms and hallways throughout the dungeons in several Zelda titles. They commonly inhabit tower-like regions and appear alongside Bumpers and pitfalls.

A Link to the Past
Hardhat Beetles first appear in the Tower of Hera, but also appear in many dungeons in the Dark World and caves on Death Mountain, and commonly appear in groups. They charge at Link wildly and attempt to knock him down pits. Their movement is erratic and their tentacles give them little friction: they are liable to overshoot Link and careen around the room, bouncing off Bumpers. Striking a Hardhat Beetle with a sword will cause it to bounce away wildly and cause Link himself to fly backwards. Due to their high defense, it is easiest to deal with them by knocking them down pits, but they will attempt to do just the same to Link. While they can eventually be destroyed by repeated blows with the sword, their high defense means it's sometimes easier to knock them down pits. In particular, the red variety can take a huge amount of damage before it is destroyed, and moves much faster than the blue variety, so Red Hardhat Beetles should be knocked down nearby holes as soon as possible. Hardhat Beetles are invulnerable to most weapons, but Bombs will stun them. They often leave more Bombs behind when destroyed.

Link's Awakening and the Oracle Series
In Link's Awakening and the Oracle Series, Hardhat Beetles are completely invulnerable to the sword, and in most cases must be defeated by knocking them down pits. They usually appear in smaller numbers however, and don't cause Link to fly back when striking them with his blade. They still retain their weakness to Bombs, but now these explosives will destroy them, rather than simply stun them. They are common in various dungeons and caves. In Oracle of Seasons, Link can use the Magnetic Gloves to crush them with metal balls.

Four Swords Adventures
In Four Swords Adventures, Hardhat Beetles first appear in the Tower of Flames, similar to their appearance in A Link to the Past. In this game they will even bounce off one another, but must again be defeated primarily by knocking them down pits. It is however possible to destroy them by hitting them with the Down Thrust using the Roc's Feather. As in A Link to the Past, they commonly appear near pitfalls and Bumpers in dungeons.

Phantom Hourglass
In Phantom Hourglass, Hardhat Beetles first appear in the Temple of Courage, and aside from a slightly shorter stature they are otherwise very similar to their previous appearances, and must be knocked down pits. The Hammer received later in the game can be used to smash the Hardhat Beetle into oblivion.

Giant Hardhat Beetle
The Giant Hardhat Beetle is a boss that appears in the Color Dungeon exclusive to Link's Awakening DX.

Trivia

 * There is a required minigame in Oracle of Ages that demands that Link defeat four or eight Hardhat Beetles in less than thirty seconds or before the minecart hits whatever item is being fixed.
 * The Hardhat Beetles in Link's Awakening, Oracle of Ages, and Oracle of Seasons loosely resemble the Blooper and Buzzy Beetle enemies from the Mario franchise.
 * The Prima guide for Phantom Hourglass referred to these enemies as Shell Beasts.