Short Answer: bi-sexual woman willing to join a couple.
Unicorn hunting? A couple that is open-minded and progressive, they’ve probably been together for a while, and they’ve decided they want to open their relationship.
The man will usually be straight, and the woman will be bi or bi-curious. They’ll be looking for another bisexual woman (unicorn) to have a relationship with them. This makes it something they’re doing together with an agreement.
They want to explore this new territory as a unit. The idea is that this will be safer. It’ll be more equal and fairer and negate jealousy. They’ll find the woman to form a triad relationship.
Looking for this mythical woman. I.e., the unicorn is so familiar that the poly-open community has a name for these people. They call them unicorn hunters.
Here are a few familiar tropes that make this behavior so complex. The couple will only want a bisexual woman because Society says it’s cool for women to explore their bisexuality, but that’s not a thing that the man should do.
The man maintains his status as the only man in the relationship with a one penis policy. The couple looking for a bisexual woman will want her to be interested in both of them.
Equally, only one man in their relationship means he can rest easy. He doesn’t have to be threatened by lady sex. That’s not real sex, right. The woman in the couple is in agreement to explore her bisexuality.
The unicorn will be expected to sleep with both of them. Simultaneously, and usually, be sexually exclusive, the unicorn will be expected to be involved equally and romantically with both partners.
The unicorn status inside the couple depends on her continuing to want to sleep with both sexes. The couple will strive, above all else, to protect their relationship as a couple.
The unicorn is not allowed to come between them.
They might only be interested in or attracted to one half of a couple. They might not be into group sex. They may not have control over how or when they start to develop romantic feelings and for whom they will still want to have a life outside of the couple.
They might stop wanting to have sex with one half of the couple, if this isn’t allowed, they have to choose between having sex with someone they don’t want to have sex with or losing someone they care about.
The unicorn is rare and often in the end the unicorn leaves.