Corporal Pig

Helmet Pigs, also called Armor Pigs or Soldier 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

 * In the promotional videos, Helmet Pigs are shown to be guards at the pigs fortress.
 * He is the only pig that doesn't display his ears.
 * Along with the other Pigs, the design for Helmet Pigs have changed in the Angry Birds Chrome Beta, they have shinier helmets with buckles on them.
 * The helmet pig might be a Medium Pig wearing a helmet, since it is as large as a Large Pig wearing a helmet. 
 * The Helmet Pig plush is the only Pig that is all beat up