From what I understand, the "unicorn" is more of a woman that is supposed to always be secondary to the relationship shared between the original couple and be completely okay with that. So if you are looking for something to be somewhat long haul and really want to care for this person in some kind of equal triad manner, then you might not have to use the term unicorn.
I could be wrong though, as even in our local poly group that term doesn't get thrown around a lot.
Welcome though! I am relatively new myself. I made an account a few years ago and never posted, and now I am back trying to be part of this community. I hope it is helpful for the both of us. :-)