unicorn definition from urban dictionary
Colloquial; Synonym for hot bi babe or HBB, often derogatory, condescending, or ironic. A bisexual person, usually though not always female, who is willing to join an existing couple, often with the presumption that this person will date and become sexually involved with both members of that couple, and not demand anything or do anything which might cause problems or inconvenience to that couple.
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.
John wants a single, attractive woman who will love and have sex equally with him and his wife, but not interfere with their marriage, move in together, help raise their kids but not have any of her own, and not have any other partners. He's looking for something that doesn't exist - he's looking for a unicorn!