It is said that after the Calamity, where Sybas made the first breach in his prison creating the World's Scar, the deities created the Guardian Races. These races, either newly made or empowered were meant to guard the breach and combat whatever of Sybas' leaked out.
Among these Guardian Races are a rare and mystical breed, created by the god of magic and evolution (Canera) himself. Canera made sure his Guardians were sturdy and blessed them with innate magic. While any race could learn magic non would ever come close to the control these Guardians have over the arcane.

A thousand years has passed since the Calamity and the already rare Monoceros have dwindled so much most people have only ever heard of them in legends. Many don't even believe they still exist. I mean, a magic wielding, one-horned creature? Surely it can't be real.

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Arynna didn't change in appearance in the rework of N'Thora. I did change the name of the species.
A little fun fact, this Guardian race was supposed to take the role of unicorns in the N'Thora setting. To that end she was designed with a single horn and innate arcane talents.
I did want to keep the rhinoceros are unicorns joke in there but I am changing the name One-horn from Latin (Uni-cornis) to Greek (Mono-ceros). I find it a more fitting name being both constellation and still a Greek species name. (Second fun fact, Rhino-ceros is Greek for Nose-horn).