Since most redheads are generally unattractive... you will find the rare sexy redhead (i.e. not albino and awkward) and you will need to express how you feel about them. As unicorns are a mythical creature, you can see how they relate to seeing a sexy redhead... it just doesn't happen very often.
Did you see that ginger unicorn over there? Man, she's smokin.
Hannah Clayton is the OG ginger unicorn.