In Canterlot's castle there is only one stained glass window, the one with all the villains that took over Equestria,the windows that depicted them being defeated had been destroyed, along with almost all the windows containing the princesses or the elements of harmony.

Some info about every "ruler" of Equestria, the ones that actually rule over something and the ones that help the villains:

Nightmare Moon:(note: Nightmare Moon is a different entity from Luna, here she only has her body) The Empress of Equestria, when she possessed Luna's body again, she allied with the other villains, through a twisted form of "friendship", she saw it herself when she was defeated by Twilight and her friends, how some allies could make the difference, so she made some to make sure that she wasn't defeated ever again. She is the calculating ruler of everything, she keeps the other rulers at bay, everybody gets their part, but all of you follow MY orders, without ME none of you would be where you are now. She gets what she wants trough fear, in fact she has managed to get the dragon lands and yakyakistan under her control. She tries to make everypony happy under her rule, but they now that this is nowhere as how Princess Celestia and Princess Luna ruled, they cared about their subjects, Nightmare Moon cares only about how loyal you are to her.

Lord Tirek: Tirek is the second ruler of Equestria, he is the one that makes the dirty work for Nightmare Moon, like threatening with his presence the cities that haven't accepted the rule of Nightmare Moon. He doesn't take magic from ponies anymore, but they know he could if he wanted to, Nightmare shares part of her magical power with him to sustain him,Tirek actually took the throne of his homeland after finding out that Scorpan never ruled, but can't leave Nightmare Moon side because of that. After he was imprisoned in Tartarus he felt hatred for his defeat, he wanted to destroy everything in that land that threw him in that pit of darkness, he was young when he and Scorpan planned taking over Equestria, so he had that immature feeling that the world was his to conquer. Now it's different, after many moons he realized that his destiny could have been great, if only he had stayed and listened to his father, a great ruler, he wishes things could go back, he thought that Scorpan was a pest, but only now he realizes how much he misses his brother.

Discord: The wildcard of the villains. At first he joined Tirek because he was very petty about being defeated a second time by those darn elements, and he knew the princesses had sent Twilight and her friends to reform him because maybe his powers could be useful someday, like when they asked him to defeat Tirek, oh how the tables turned when HE helped him. He agreed to help Nightmare Moon in exchange of some land to make to his liking, a little paradise of chaos, and he got it. He feels a little guilty because he actually liked the company of Fluttershy, but it all comes over to those who don't have to control him. Chaos can not be controlled.

King Sombra: The ruler of the Crystal Empire. He is the one who has the most sympathy for ponies, after all he was raised by them, there was kindness in some of them, and he saw that. His hatred was toward the conflict that caused both the crystal ponies and the umbrums over the land that would later be the empire, and how the umbrum were treated, imprisoned in crystals, he wanted the crystal ponies to amend their errors. He wanted to destroy the Crystal Heart, the artifact that hurted him and would never allow his people to be in the empire. When he returned, he made sure to make the same mistakes that he did in the past, he destroyed the Crystal Heart and made a truce between the Crystal Ponies and the Umbrums, he doesn't involve much in other matters in Equestria. He is used by Nightmare due to his vast knowledge of dark magic.

Queen Chrysalis: The ruler of the Everfree Forest. She finally has what she wanted, all the love she can consume and a part of Equestria where she can do whatever she wants. She lives in luxury in her hive, nobody is above her, she is the feared and respected leader of the changelings. The changelings are of great use to Nightmare Moon rule, they are a great military force, and they can change into whatever creature they want, perfect for infiltrating and catching rebels.

The Storm King, Starlight Glimmer and the sirens (not in the picture): The helpers of the rulers, the storm king helps with the military and is the "boss" at Cloudsdale, Starlight is the controlling mayor of Ponyville,one of the most loyal subjects of Nightmare Moon, and the sirens help intimidating the people of Equestria.