Helmasaur

Helmasaurs (ヒップループ, Hipuruupu), also known as Iron Masks or Hiploops, are recurring enemies in The Legend of Zelda series. They wear iron masks which must be destroyed or forcibly removed to damage them, or at least make it easier to defeat them. Their weak point is their behind.

A Link to the Past
The Helmasaurs, in A Link to the Past, are small dinosaur-like reptilian enemies that must be attacked from behind. Alternatively, Magic Powder can be used to transform it into a defenseless Slime. There is also a boss by the name of Helmasaur King.

Link's Awakening, Oracle of Ages and Oracle of Seasons
They are called Iron Masks in Link's Awakening, Oracle of Ages and Oracle of Seasons. It wears a mask of iron to protect its vulnerable body, and can easily overtake a would-be hero if encountered in large numbers. Like other incarnations, its backside is unprotected by its mask, so a weapon like a sword hitting its backside will result in quick death.

The Iron Mask is a relatively easy enemy for Link to defeat. Link can either attack it from its back, or rip its mask off with the Hookshot, the Magnetic Gloves, or the Switch Hook. This latter method will often result in some damage, however, due to Link switching positions with the mask. Without its mask, Link can attack the Iron Mask from any direction:
 * In Link's Awakening, the Hookshot could be used to pull the mask off of the creature, rendering it incredibly easy to kill without its main defense.
 * In Oracle of Ages, this same thing could be done with the Switch Hook and Long Hook.
 * In Oracle of Seasons, magnetic disruptions from the Magnetic Gloves could also rip the mask off the creature.

Majora's Mask
In Majora's Mask, they are called by their original japanese name, Hiploop, and are encountered in Woodfall and Stone Tower. The Hiploop doesn't always wears a mask, and is looks more like a large, aggressive, insect-like enemy. Once they spot Link, they will charge at him and attempt to knock him off of platforms or walkways. To defeat these enemies, Link must use his shield to defend against their charge attack. This will knock them backwards, allowing Link to counterattack.

To defeat the Hiploops that wear an iron mask on their head, Link must first remove the mask using the Hookshot before being able to attack it in the usual manner.

The Minish Cap & Four Swords
Known as Helmasaurs in The Minish Cap and Hiploops in Four Swords, they are small, red creatures with a pointed helmet, more resembling their Link's Awakening and Oracle series' incarnation. They will charge at Link and can only be damaged from behind. In The Minish Cap, the helmet can be removed using the Gust Jar making it susceptible to sword attacks and less of a threat.

Twilight Princess
In Twilight Princess, Helmasaurs are found in the Lakebed Temple, City in the Sky, Cave of Ordeals and Hyrule Field. In addition to waiting for them to charge, dodging and slashing their backside, Link can target them with the Clawshot to remove the "helmet" and reveal an attackable creature. While there are many ways to remove the "helmet", bombs are not an effective method.

Helmasaurus
The Helmasaurus is a Twilight Princess original enemy that appears in the City in the Sky.

The Helmasaurus is a differently colored and much larger version of the Helmasaur. Like the Helmasaur, it also attacks by ramming into Link with a forward charge. The iron plates of a Helmasaurus can't be removed like those on a Helmasaur, so getting behind and striking at its vulnerable backside is the only way to defeat them.

Trivia

 * The Helmasaurus from Twilight Princess may be an adult form of the regular Helmasaur.
 * If one looks closely, the Helmasaurs from Twilight Princess appear to have flat, grinding teeth, implying that they are herbivorous animals.