“Baby do you dare to do this
Cause I'm comin' at you like a dark horse”

Katy Perry – Dark Horse Lyrics on LyricsMode
EXPLANATION
She is warning that she is much better than he expects.
EXPLANATION
She is warning that she will come unexpectedly to him.
EXPLANATION
'Dark Horse' is reference to the apocalypse It's a warning that once you choose her you choose the 'End'.
EXPLANATION
Dark horse: a person who has interesting qualities or abilities that most people do not know about.