Unicorn, aka Hot Bi Babe or HBB, is a single bisexual (and hot!) woman who is looking to form a polyfi triad with an existing male-female couple, most often a married one. Unicorns are easygoing partners because they don't have any needs of their own in what comes to a relationship. They are totally happy to hang on to whatever nuggets of love and affection the original couple fish out their way. They are not usually very old and hence might have economic issues, and they are only too happy to move in with the couple in very early stages of the relationship. And of course they are willing to help around the house and with kids and whatnots, after all, they are getting free room and board.
Should somebody ask, unicorn is the live-in nanny/struggling student renting a room. They are never introduced to the family, invited along on social outings or holidays. The original couple maintains primacy and social approval, whereas the unicorn has to face constant pity from people who don't know she isn't single and comtempt from people who equate her with a homewrecker. Unicorns don't have kids because that would seriously mess up the dynamics. They can help bring up the original couple's kids, of course with no legal rights to them whatsoever. Unicorns are also easily disposable should they become cumbersome or needy. If something goes wrong in the relationship, it is the unicorn who isn't emotionally mature enough/doesn't respect the original couple/doesn't know how to share/is needy/is clingy/is unavailable/isn't committed enough etc.
Unicorn care is easy as long as you remember a few easy rules;
1) Original couple goes first. Always.
2) Unicorns are not really people, they are emotional and sexual resources to the original couple. Sex is only allowed in threesomes, and no individual relationships between the unicorn and either member of the couple should ever develop as not to threaten the cohesion of the original couple (see point 1).
3) If something's not working, play the 'How can you not be grateful for all we have done for you?' card with your unicorn, and if that isn't enough to scare her off, call her a homewrecker.
The above is a completely satirical account of what gives the term 'unicorn' or 'unicorn-hunting' such a bad aftertaste to some people in the poly community, and is in no way meant to reflect on what people seeking committed triads are actually like.
Me: bi female in my twenties