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A King is typically the title of a hereditary, male monarch of a nation, region, or state. The spouse of a king is usually called a queen, but is rarely given a portion or shared rulership. A king may be she spouse of a female monarch called a queen, but that is rare.
King of Gnomes (although the position is not technically hereditary).
Kings of the Horde
The Horde does not typically call their leaders kings, even if they are male monarchs. The title is largely historical from a group that joined the Horde and used that title previously.
The leader of the Horde is called Warchief, and races such as the tauren and trolls use chief or chieftain as their titles. "Regent Lord" is the blood elven regal title, while the goblins are ruled by a slew of trade princes.