Helmasaur

Helmasaurs are small, dinosaur-like enemies. They wear armor/helmets/masks which either must be destroyed (or forcibly removed, as in Twilight Princess) to damage them, or at least make it easier. 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 you 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.

Quote from The Legend of Zelda: The Minish Cap Helmasaur figurine: "Appears in various areas. Their fronts are protected with metal masks, but their backsides aren't! Pop off that mask for an easy battle."

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, you can target them with the Clawshot to remove the "helmet" and reveal an attackable creature.

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 you 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.