Helmasaur

Helmasaurs, also known as Iron Masks or Hiploops, are recurring enemies in The Legend of Zelda series. They are almost always depicted as saurians (either dinosaurian or avian) wearing 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. They are also weak to the Hookshot; they are defeated in one hit with it. 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. They wear masks of iron to protect their vulnerable bodies, and can easily overtake a would-be hero if encountered in large numbers. Like other incarnations, their backsides are unprotected by their masks, so a weapon like a sword hitting their backside will result in a quick death.

The Iron Mask is a relatively easy enemy for Link to defeat. Link can either attack them from their back, or rip their masks 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 their masks, Link can attack the Iron Masks from any direction:
 * In Link's Awakening, the Hookshot could be used to pull the masks off of the creatures, rendering them incredibly easy to kill without their 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 Glovess could also rip the masks off the creatures.

Majora's Mask
In Majora's Mask, they are called by their original Japanese name, Hiploop, and are encountered in Woodfall and Stone Tower. Hiploops don't always wear masks, and they look more like large, aggressive, insectoid enemies. 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.

Four Swords and The Minish Cap
Known as Hiploops in Four Swords and Helmasaurs in The Minish Cap, 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. The helmet can be removed using the Magnetic Gloves or Gust Jar in Four Swords and The Minish Cap respectively, making them 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 are differently-colored and much larger versions of the Helmasaur. Like the Helmasaur, they also attack 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 their vulnerable backsides is the only way to defeat them.

Trivia

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