Those who opposed the Nightmare’s takeover where enslaved and broken, destined to be servants and playthings to more loyal subjects. Particularly important rebels, be they leaders or person’s of great power, are personally claimed by Nightmare Moon to be part of her harem. The crown jewel of the harem is Celestia herself, Nightmare’s sister and first conquest in her rise to power.
 
Nightmare Moon has since spent a lot of time and effort training her sister into being an obedient pet, and takes great pride in showing her off to guests.

Twilight’s capture: Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7(final)

Starlight’s Capture: Part 1, Part 2, Part 3(final)

Meet the Harem: Celestia, Cadence/Shining, Zecora, Chrysalis/Sombra, Twilight/Starlight

Harem Fun: Don’t Squirm, Guard Perks, Guy Time, Hot and Heavy, The Stretchening, Student-Teacher Relations, While the Mistress is Away

Beyond the Harem: Rebels Fate

Zecor-iture: Fountain, Lamp, Vase