More Millenia stuff. I have no idea what I’m gonna do with her, but I’ll figure out something.  
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So yes, the pony here was possessed by her spirit. Using an Illusory spell, she can appear to be 16 again (or any age younger than her current one) when necessary too. What did her body look like 5,000+ years ago? shrugs  
I guess this is a headcanon where Necromancy was a thing, a long time before Equestria was founded.  
Maybe in some kinda lost civilization? No idea yet, but stay tuned!