I found something interesting. There was a character in the increasingly popular game Overwatch named "Sombra."
I always thought the name Sombra meant Somber as in a form of sadness. But from Overwatch's Sombra's accent and her ending comment "Adios" lead me to understand that Sombra meant something else in Spanish, so I looked it up as Spanish and it meant this.
Sombra Meaning in Spanish
Shadow or Shade. In other parts of google it seems Sombra also refers to a dark/shady part of a ring in a bullfight. Anyway I accept the Shadow/Shade part of it and the bullfighting part kind of hinted me into thinking it also means a meaning for more covert/hidden means (the Overwatch character's abilities, background and personalities all hint this out as well). So it doesn't just mean shade and shadow but can mean an adjective for covert/hidden things done in the shadow.
Could this mean King Sombra can possibly be Spanish or Latin American as a pony? Not all Spanish/Latin Americans have an accent but it wouldn't hurt to give him an accent too. Maybe sound like Zorro or Bane (Batman).
Somber can still stand as a meaning though as the Crystal Ponies were very depressed and somber from his influence.