Corporal Pig

Helmet Pigs (a.k.a. Soldier Pigs, major pigs, armor pigs, knight pigs, nazi pigs, or cake mix piggies) are pigs that are 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 also serve 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, Billy the Pig, and Cave Explorer. Also, those bosses are achievements. The Helmet Pig first appears in Poached Eggs Theme 1-21

Trivia

 * In the promotional videos, Helmet Pigs are shown to be guards at the pigs fortress.
 * Helmet Pig 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 Large Pig wearing a helmet, since it is as large as one wearing a helmet. 
 * The Helmet Pig plush is the only Pig that is all beat up
 * The helmet the Helmet Pig is wearing resembles a nazi german helmet used during World War II.
 * In the Chrome version the Helmet Pig's helmet may be made of metal which may explain why the helmets are shiny.