Conventional wisdom and common belief are that Princess Platinum's father was King Bullion, and a wise old king was he, and that Platinum became Princess-Regent when Bullion felt that he was no longer able to govern.
This is a falsehood which was spread centuries ago by parties unknown (either unwittingly or deliberately) and is repeated nowadays by most everypony.
Tracking down primary source historical records is difficult at the best of times, as Canterlot guards its secrets jealously. But what we find, if we really look, is that there was an historical Bullion, and he was a duke, not a king; specifically he was Platinum's maternal grandfather.
The historical Duke Bullion was very much a product of his time but appears to have been downright progressive compared to the majority of unicorn society. Yet Platinum started off as a tiny tyrant, which indicates that her parents had more sway over her upbringing than her grandfather.
So, who were her parents? Princess Platinum's mother appears to have been a Queen Silver, (somepony who was apparently less open-minded than her father) and this individual either died in childbirth or shortly after, leaving Platinum to be raised, presumably, by her father the Unicorn King.
And what we eventually uncover (after many library trips and much frustration) is that there are no records of the Unicorn King; not his name, not his parents, not his entire lineage prior to Princess Platinum. It isn't that records weren't kept; it appears that they were deliberately expunged, with some documents being modified and other documents deliberately destroyed.
The Unicorns kept meticulous records of everything and everyone (including the horrors in the crystalline labyrinth beneath what is now Canterlot)—but of the historical Unicorn King, no records, no name, no trace.
The centuries-old Hearth's Warming Pageant makes clear that the Princess Platinum in the play is the selfish and bellicose leader of the unicorns (thus necessarily Princess-Regent) and that she is the “daughter of the Unicorn King”—but the king's name goes unmentioned because all traces of his identity and lineage were deliberately expunged at some point prior.
Somepony wanted all traces of this particular ruler erased, and they were willing to erase his entire family tree to do it. And circumstantial evidence suggests that this act might have been ordered by Platinum herself.
These circumstances leave us all with a singular question: “Why?”
