"The line, "My huckleberry friend," is often thought to be a reference to Huckleberry Finn, a character in Mark Twain's book Tom Sawyer.
However, in his autobiography, Johnny Mercer said it was in reference to a childhood friend of his. He used to pick huckleberries with him down by a lazy river near his home in Georgia."

https://www.songfacts.com/facts/henry-mancini/moon-river?fbclid=IwAR3P7Xkof5A3o1W_Q-hj6tvcCBhWRvvGK0mnAVoU0-Cxi05eBFZ01IFUeVw