Chapter 58: Sacrifice
Bloodborne unicorns were very rare and special Spirit Beasts. They had been hunted to almost extinction because of their horns. They were special for a number of reasons. First, they were extremely intelligent and were able to communicate with others regardless of species. Second, they mutated very easily. Especially if they were still in the womb.
So, pregnant mares would look for natural treasures in hopes that their progeny would be stronger than them. A special hormone that was only produced during their pregnancy made them extremely sensitive to natural treasures. But this was also their curse.
Their bloodline was too mutated. Because of this, young foals would often die. Third, and perhaps the most unique thing about them was their bloodline memories. Unlike other Spirit Beasts, they pass not only abilities and skills but also literal memories to their descendants.