What is the rarest type of unicorn?

Excuse me, I'm curious to know, what exactly is the rarest type of unicorn that exists in the realm of mythology and folklore? Is it a specific breed, perhaps with unique markings or characteristics that set it apart from the rest? I've heard tales of many different unicorns, but I'm particularly fascinated by the rarest and most elusive of them all. Can you shed some light on this intriguing question?