Prince Yeun Nan Hung Lit of the Jin Empire was not a real historical person, so Jin Yong had to devise a name for this character that he introduced in LOCH. I wonder if in giving the character this name, Jin Yong didn't look to a real historical ruler who would appear as a character in another novel (DGSD) that Jin Yong would later write for inspiration: Yeh Lut Hung Gei of the Khitan Liao Empire.

It's pretty easy to see how Jin Yong might have accomplished this: just replace the Liao royal surname of "Yeh Lut" with the Jin royal surname of "Yeun Nan," recycle the "Hung" from the first character of the given name, and then swap "Lit" for "Gei."

That's probably how Jin Yong did it.