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The enigmatic Night Guard, under direct control of Princess Luna and no one else, has made very few public appearances so far. Supposedly composed entirely of Batponies, if Glimmer/Batmom is any indication, their involvement in the shenanigans of the princesses - friends and foes – is scarce and never documented or directly observed.
There are rumors that they secretly keep Equestria safe during their Night, that they are the Mares in Black, that they are Ninjas. None of this is confirmed so far.
The only open appearance of a member of high rank of the Night Guard would be the Night Guard Captain, so far unnamed, who seems to run things a little TOO competently. Shining Armor seems to have been aware of there being a separate Night Guard Captain, but never mentioned him nor does he even know him by name.