Boastful, self-aggrandizing songs really occur in all genres, but it's particularly common for rappers to rap about how they're badass/what they can do, about their possessions, homies, and stuff, about who they are, or a combination of all of the above. Almost every other Hip Hop song is like this. Compare Bragging Theme Tune and A Wild Rapper Appears. If sung by a antagonistic character, can be considered a Villain Song.