I've lived most of my life in the Southern U.S., read a whole lot about it and other things, and I've *never* encountered "huckleberry friend" outside of the song.

Or "apple friend," "flower friend," "cotton friend," etc.

Why not just accept that Andy Williams made it up?

I50 years ago a common frontier idiom was "That's a huckleberry above my persimmon." It meant "That's beyond my reach, ability, etc." Why? Who knows?

Similarly, "huckleberry" is sometimes used as a humorous synonym for "a man or a boy." Whether that came before or after the 1884 novel, I don't know. But I certainly remember the late, great N.Y. Yankees announcer Phil Rizzuto, a New Yorker through and through, more than once asking "Who *is* that huckleberry?"

As for H. Finn, I've never heard of anybody else named "Huckleberry." It's a fact, though, that Samuel Clemens had been threatened with a lawsuit, by a certain Eschol Sellers, for including a character of that name in a previous novel. Compelled to change the character's name in later printings, Clemens/Twain changed it to "Mulberry Sellers."

Long before "Huckleberry Finn," Twain wrote that huckleberries were a novelty to him when he moved to Connecticut. He said they didn't grow in Missouri.

(Most of this comes from a 1971 article by James L. Colwell of the U. of Colorado.)