Angry Bird v Pig
Yes, we can understand why the birds are angry that their eggs have been stolen. But why pigs instead of, say, foxes, as the egg-stealing enemy?

According to an interview with Ville Heijari of Rovio, which developed the game, the game designers had the bird characters ready in early 2009, but didn't have a concept for the birds' opponents yet. Eventually, they settled on sick-looking green pigs because of a big scare going on around the world at that time: fears of swine flu.