She’s a Twonicorn, an incredibly rare but not unheard-of mutant Unicorn blessed with two magical horns, rather than one. Only one in seven Twonicorns are lucky enough to be able to use magic with both horns, and she’s one of the lucky ones. Each horn can focus magic of different wavelengths and cast spells independently or mix different spells and elements for stronger effects, but both horns draw from the same magical source. While she can cast more spells than the average Unicorn in a minute, doing this exhausts her limited stamina much faster. Also, her mane and tail colours don’t match due to a rare genetic condition. And her eye colours don’t match due to a different but rare genetic condition that’s less rare than the hair thing. If this wasn’t Equestria, a country full of (mostly) universally kind and accepting ponies, she would likely be considered some sort of freak for having three incredibly rare genetic mutations in one body.
Her Cutie Mark is two Brilliant Rose diamonds, it means duality and light, and it makes her good at casting two beneficial spells at once. Not quite as talent-enhancing as a Cutie Mark in magic itself, but not far off.