Helmasaur

Helmasaurs are small, dinosaur-like reptilian enemies. They wear armor/helmets/masks which must be destroyed (or forcibly removed, as seen in Twilight Princess) to damage them, or at least make it easier to defeat them. Their weak point is their behind. The Helmasaur's Japanese name is Hiploop (ヒップループ), so this is the same enemy as the Hiploop from Majora's Mask.

A Link to the Past
The Helmasaur in A Link to the Past 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.

The Minish Cap
In The Minish Cap Helmasaurs are small, red creatures with a pointed helmet, more resembling Iron Masks than traditional Helmasaurs. They will charge at Link and can only be damaged from behind. The helmet can be removed using the Gust Jar making it susceptible to sword attacks and less of a threat.

Twilight Princess
Helmasaurs make a return in Twilight Princess and 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.