Originally this was a name given to an impossible non-existing mythical beast: an attractive bisexual woman who would not impede upon a heterogendered couple’s relationship and at the same time be monogamous only to that couple.

I’ve decided to reclaim unicorn to describe a single poly-identified man who seeks to develop playful friendships with other poly guys, couples, and packs without necessarily requiring a commitment from those parties; available to play without interfering in established relationships but ultimately seeking to attach strings where there is mutual interest.

See more: http://www.urbandictionary.com/define.php?term=unicorn