Dark Horse" combines the genres of trap and hip hop, replicating what has been described as a "Southern rap-techno mashup". The track features a minimal production, with a "seductive" and "mature" tone to Perry's vocals, while Juicy J is featured on the song's intro and rapped bridge. Perry, in interviews, said she wanted the song to have a "witchy, spell-y kind of black magic-y idea", so she wrote it from the perspective of a witch warning a man not to fall in love with her because if he does, she will be his last. The song was part of a competition sponsored by Pepsi in which fans could vote via Twitter on whether they would prefer either "Dark Horse" or "Walking on Air" to be released as the first promotional single from Prism.