Corporal Pig

Helmet Pigs, also called Armor Pigs, are depicted as pigs wearing stone helmets. They are the third strongest pig variety in the game, and serve as the chief enemy in the first set of levels in each theme. They're also seen serving as protectors to the King Pig and Mustache Pigs. Helmet Pigs are very much alike to Mustache Pigs in game ways, not actual looks. They have bosses, Herr Helmet, Hovering Helmet, Hardhat Hidalgo and Billy the Pig. Also, those bosses can also be achievements. The Herr Helmet first appears in Poached Eggs Theme 1-21

Trivia

 * The Helmet Pig Plush is only available in a form where it is already damaged.
 * Along with the other Pigs, the design for Helmet Pigs have changed in the Angry Birds Chrome Beta, they also have shiny helmets.

It's the only pig that does not show its ears. (The ears are under the helmet.)