Proof that it is just one mare:

Mainly bringing me to the wedding episode where Luna is nowhere really to be found with Cadence or Chrysalys. Anywhere. So, still not convinced? "What did I miss?" Direct quote from Luna herself. Also, she is proven to be Nightmare Moon. What stops her from being Chrysalys as well? Nothing. You see, each mare has her own pros and cons. If NIghtmaremoon was defeated, why was Luna alive? Exactly.

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Would that mean that the Changelings are actually mutated batpoines?

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It could also be that they were formed from the same type of dark magic, or that Chrysalis is a past version of NMM, while Luna was just a vessel for them of some sort.

4638019 YES, OMG