Katy Perry's "Dark Horse" was co-written by Sarah Hudson, who is a singer-songwriter and a member of the Pop group Ultraviolet Sound. Though Sarah isn't related to Katy (whose real name is Katy Hudson), she is the first cousin of another famous person with the same name, the actress Kate Hudson.
Who writes a song about a name they found in a phone book? That's just one of the everyday things these guys find to sing about. Anything in their field of vision or general scope of knowledge is fair game. If you cross paths with them, so are you.