You have been given the correct meaning, in the most polite of ways. Yet you continue to attack people who are trying to help you. Maybe you should learn to accept your mistakes, and be grateful for help?
What makes you doubt that this meaning is correct as you seem to be persisting with your incorrect assumption?
What do you think it means exactly?
The commonly accepted definition (one could almost say universal - hah!) of a unicorn is something so rare and precious as to be virtually unobtainable.
Plenty of posters on this thread have indicated that they already have the bass they consider appropriate for all kinds of music - ie universal, myself included. Most of these are not unicorns and are in fact generally very common - P-bass, J-bass etc...