Addar
Addar is a minor Darklord from the Ravenloft setting. Originally a proud and haughty unicorn, his arrogance ultimately led to his downfall when it led to his causing the death of an elven paladin-princess who was destined to be a great champion for the powers of light, an act that caused him to be swallowed by the Mists of Ravenloft.
Addar was first mentioned as the father of the race of Shadow Unicorns in the Ravenloft Monstrous Compendium III for Advanced Dungeons & Dragons 2nd edition. He would receive a full writeup of his own in the Books of S #5: The Book of Sacrifices, making his status... sort of canon. Whilst he himself is stated as existing in the Shadow Unicorn's 3e update in Denizens of Darkness/Dread, his stats and backstory are technically made.