Finally got a chance to binge watch the last three episodes as I was, ironically, sent home early today after working all weekend.
As an unrepentant Earth pony promoter, I am pleased to see that there's no huge fandom outcry that Meadowbrook was of the hornless variety of equines, and I'd like to add my own two bits into the hat.
First, she was an enigmatic healer that wore a mask, appearing for just a single night and "magically" curing all ponies of their ills. I can see why contemporary ponies of her era may have attributed such abilities to an active magic wielding unicorn if they could not see her face.
Secondly, the sheer amount of time and evolution of language means that any Modern Ponish texts (which I'm assuming is what they used to teach at Gifted Unicorns) need to be taken with a lick of salt. Given the magical scholarly focus of Equestria, the Old Ponish term for "wise" or "learned" could have been attributed to "mage" when translating (hence "Mage Meadowbrook"), and therefore to "unicorn" as well by an innocent mistake. Few primary texts from over a thousand years ago can be reliably used without addressing both issues in translation and accounting for contemporary beliefs and biases.
Finally: was it ever stated outright that she was an Earth pony to Twilight and Fluttershy? The audience got to see so in the flashbacks, but the ponies weren't privy to such narrative devices. While there are a number of circumstantial pieces of evidence to this (Cattail's tribe; Meadowbrook's adobe and method of healing e.g.), none of these are definitive, and the reason that Twilight does not comment on the disparity could be that she is simply not aware of it. I'm sure that finding the laboratory home and journals of a respected healer would be of significant academic interest to Twilight - being able to correct the history books regarding Meadowbrook's tribe would have certainly brought out the adorkable Twilight that we love had she known.
Finally finally: the magic mane fiasco in Twilight's castle reminded me of a scene from one of @Novel Idea's stories. That was awesome!