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.

A Link to the Past
By using a Bomb or the Magic Hammer, their helmets can be smashed to render them defenseless. 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. 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.

Twilight Princess
Helmasaurs make a return in Twilight Princess and are found in the Lakebed Temple 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. There are also larger Helmasaurs named Helmasauruses.