Ninja Pigs are pigs that disappear with the background. The bird guns are programmed to shoot whenever they see a pig coming. But when it disappears, the birds won't shoot.
Single click, and the pig will disappear.
Double click, and the pig will throw ninja stars.
Before v.1.0.3, the Ninja Pig didnt have a mouth covering, but it had a ninja mask.
The Ninja Pig isn't in any cutscenes.