Sir X. Checker is a male anthropomorphic dog.
The elderly and sensible Sir X. Checker is a member of the Mintavian aristocracy and the country's royal treasurer. In 1982, he traveled to Duckburg to enlist the help of Scrooge McDuck and his gold-sensitive sense of smell to solve a particularly tricky counterfeiting problem. He then accompanied Scrooge throughout his investigations.
Behind the scenes
The King only ever appeared in 1982 in The Counterfeit Caper.
The story does not say what the "X." in his name stands for, though it is usually reckoned to be Xavier.