Are Centaurs rare? In most of this crazy world of ours, yes. Most haven't even seen one!
Are Unicorns rare? Oh my goodness! Now that's a whole ten steps beyond Centaurs! They're rare indeed, rare enough to be considered not real, just a fairytale legend by pretty much everyone.
Now pay attention, how about this? Just how rare would a Centaur that's an Unicorn be? Rarer than winning the lottery for a million years in a row, I tell you! However they exist! Well...at least one of them exists.
No one knows the whens, the hows, or the whys (not even herself) but Spring (she named herself after her favorite Season) is real, and both a Centaur and an Unicorn indeed. No tricks here! Only one problem...she can't remember where she's from, or if more like her exist. She doesn't even remember ever having a childhood! All she can recall is having woken up under a tree about seven hundred years ago. Did she appear there out of the blue? Did she wake up with the longest running amnesia episode known to man and beast? Was she actually born from that tree? Spring doesn't have the slightest idea, but there's something she does know: She has magical powers!
Not your average set of magical powers though. No fireballs fly from her hands, no lightning from herfingertips, and surely no deadly beams from her horn. Spring's magic...well...makes others feel good! Her healing magic is by far her biggest feat. Spring can't bring the dead back to life, but can heal any living being back from the verge of it no matter how severe the wound, and cure not all, but most known illnesses, and cleanse any present poisons or derivates. Her healing is so great, it will even close small wounds, ease light pains, and heal light illnesses (like a common cold) by just standing near Spring, even if she's casting no magic at the moment. A secondary effect of her magic and pressence, at least on those succeptible to it, is a general and unexplainable feeling of happiness and joy. For some strange reason however, Spring's healing doesn't work on herself, only on others. Her list of skills also include, among others, the ability to speak with any animal, and instantly make small plants grow. Of course, having the body of a mare from her waist down make her extremely fast with outstanding endurance to boot, and able to perform powerful kicks with her hind legs (not that she'd ever kick anyone!). Another of her "powers" consists in breaking into song without warning, and not always for a valid reason. She can survive (healthily) by just drinking clean water.
Spring is first and foremost a pacifist and a lover of nature. One of her goals is find out who she is and her true origins, but she sidetracks all the time in order to do what makes her the happiest: Help others. However she's too trusting and naive, and is often taken advantage of. Spring is friendly and open to a fault, and isn't the slightest shade of shy or subtle about it. All in all, the friendliest Centaur Unicorn you've ever met! Well...and the only one, probably.