The term is often used to be dismissive of a couple seen to be only superficially polyamorous. Because of the demands that this type of couple places on the woman (that she be single and not take on any additional partners, and become involved with both members of the couple equally, and often "complete" their family as a surrogate mother and housekeeper and/or breadwinner and not do anything that may threaten or disrupt the existing couple), many in the poly community call this type of woman a "unicorn", as in mythical and not likely to be found, even though there are plenty of bipoly women around.
Sometimes the unicorn is expected to not develop any emotional attachment and is strictly there for a sexual relationship (equally distributed to both members of the dyad) and/or is prescripted as a secondary. This term is used as a reminder that bipoly women are people with their own desires, needs, and pre-existing lives, and not fantasy figures or pets. See related prescriptive vs. descriptive.
“That girl is a ten for sure.” “No bro, she’s not a ten - she’s beyond a ten. She’s my unicorn.”
Later, they evolved into two species:
1. the white unicorn (of horselike appearance with one beautiful horn on the center of its head)
2. the pink unicorn (of unknown appearance, as it is invisible)
Walt: Oh, it's lovely! Well, if I draw it as a white horse with a horn...
Scott: I don't believe in unicorns. Besides, it's pink.