Sack Pigs (Also referred to as Scarecrow Pigs) are special pig variants that first appeared in Haunted Hogs, an episode from Angry Birds Seasons. They're crudely made Rag Dolls that behave like an enemy the player must destroy to finish the level.
A Sack Pig is shown when you fail a level in Haunted Hogs. (Albeit torn up)
Despite being mostly used for Halloween levels, 3 shrunken Minion Sack Pigs are featured on Tropigal Paradise's 1-7, and 5 more are used as the only enemies in Fairy Hogmother's 1-7.
An unused small variant of the Minion Sack Pig can be found in the editor of older versions, although it doesnt have a sprite.
The 5000 Points form of the Sack pigs is a desaturated version of the normal pig's.
In The World of Angry Birds Official Guide book, Foreman Sack Pig is described as a pig wearing a Mexican skull mask, while Corporal is described as a pig wearing a hockey mask.
Them, alongside the Demon Rubber Ducks in Friendship Day and the Weapons in Ahtisaari Days are the only enemies in the game that arent pigs. (These just look like them)