Patronouses represent the witch's or wizard's soul, so it makes sense that Muggleborns would have patronouses relating to the Muggle culture. One Muggleborn might have, say, a dragon from Game of Thrones, and another might have Pikachu, and another might even have the Hulk as their patronous.
But fictional things aren't all that make us feel safe. Music can impact that a lot too.
So I like to think that some Muggleborn might have, oh, say, Mother War from the My Chemical Romance The Black Parade album, or the skull for All Time Low.
So the Muggleborn finally successfully produces a patronous, and it's Mother War. Mother freaking War is their patronous, and everyone else is absolutely terrifed and another Muggleborn in the room recognizes it and screams out "WELCOME TO THE BLACK PARADE."
And all the others are so confused because they don't know anything about Muggle culture, and Mother War is terrifing, so they're scared and confused and the Muggleborns are laughing their asses off.
A/N: Hey guys, it's me! So I got this idea from a tumblr post that said basically the first paragraph. And since I really love music, that idea came to me. If you like this idea, please vote, and feel free to share your ideas on the matter! (Also, the picture that's on the side is Mother War. I felt the need to include it.) ((Also, the patronous could be anything. Use your imagination. Replace it with whatever. I just wanted to include the example.))
~Ariella