The King of the Golden River is a magically powerful gnome of disputed existence.
The King of the Golden River is, according to the eponymous fairy story, a powerful gnome who rules over the Golden River, rewarding the unselfish by allowing them access to the River's titular golden boon. The story saw him give a selfless boy called Gluck magical water which would activate the River's magical gift and make him rich, only for Gluck to give all of the water to thirsty travelers he met on the way. This had in fact been the King's final test, and he gave Gluck the gold anyway. He also magically transformed Gluck's Brothers into black rocks in punishment when they attempted to take gold from the River without his say-so.
In 1958, Huey, Dewey and Louie Duck and their uncle Donald impersonated the King, believing him to be fictitious, to teach Scrooge McDuck a lesson about selfishness, having figured that the "trick" to controlling the flow of gold was nothing magical, but simply a matter of whether or not some hot springs were connected to the main riverbed. However, in the course of this adventure, one account states that one of the children briefly caught a glimpse of a gnome identical to the way Scrooge had imagined the King…
Seemingly coincidentally, the Meso-American King Dondorado was nicknamed "the King of the Golden River" in his lifetime, and his cursed ghost was encountered by the Ducks in circumstances very like those of the King of the Golden River folktale, complete with magical treasure-granting waters which must be given to thirsty travelers as a proof of virtue.
Behind the scenes
As a fiction, the King of the Golden River is mentioned in the 1958 Carl Barks story Uncle Scrooge and the Golden River. The story remains ambiguous over whether the gnome really existed, but a 2009 illustration by Don Rosa seems to confirm that he did.
He is, of course, based on the character of the same name in John Ruskin's The King of the Golden River. The original story goes into more detail about him than his relayed in the story: Gluck's act of selflessness is to help free the King from a golden mug in which he had been magically trapped by "a stronger King". He is described by Ruskin not as a gnome but as a dwarf in Ruskin's text; illustrations for the story usually depict him as more gnome-sized than dwarf-sized, but do give him a beard.
In the Guido Martina-written Great Parody of Ruskin's story, Donald Duck and the King of the Golden River, Dondorado is the King of the Golden River, and the detail of his being freed from a golden cup in which he had been imprisoned is restored. No narrative link is drawn between the Great Parody and the previous Barksian allusions to the folktale, and as such, the Wiki must default to treating the two as unrelated.