The word "Sunlord" originated from a character called
Sunny Starscout, the main protagonist of Generation
5 of My Little Pony. The word comes from the first part of
Sunny's name, "Sun" and combined with the word "Lord" as in Overlord.
Sunlord is a term commonly used to describe
Sunny Starscout whenever she is forcing others do to her own things for her own selfish and idealistic needs, much like a dictator. She usually disregards anyone else's opinions and shows lack of consideration to push her own agendas.
The term Sunlord can also be used as an adjective, specifically to describe
Sunny Starscout or in rare cases, other characters.