This list probably has someone missing. If you think someone got left off, tell me.
Roy Acuff - The King of Country Music
Elliot Adnopoz - Ramblin' Jack Elliot
Dave Akeman - Stringbean
Rex Allen, Sr. - The Arizona Cowboy, The Voice of the West
Bill Anderson - Whispering Bill (Anderson oft times added)
Eddie Arnold - The Tennessee Plowboy
Chet Atkins - Mr. Guitar, The Country Gentleman
Gene Autry - The Singing Cowboy
William Boyd - Cowboy Rambler
Leon Eric Brooks - Kix (precedes surname in place of Leon Eric)
Lester Burnett - Smiley (preceded Burnett in place of Lester)
Johnny Bush - The Country Caruso
William "Bill" Carlisle - Jumpin' (preceded Bill)
John Carson - Fiddlin' (preceded John)
Anita Carter - The Appalachian Angel
Maybelle Carter - Mother (preceded Maybelle)
Johnny Cash - The Man in Black
Jack Clement - Cowboy (preceded Jack)
Donnell Cooley - Spade (preceded Cooley in place of Donnell), King of Western Swing (ultimately this title would settle on Bob Wills)
Dale Cooper - Stoney (preceded surname in place of Dale)
Lloyd Copas - Cowboy (preceded surname in place of Lloyd)
Billy Wayne Craddock - Crash (billed as Billy "Crash" Craddock)
Jimmy Dickens - Little (preceded his first name), Tater
Raymond Earwood - Mundo
Frenchie Edwards - Stoney (preceded surname in place of Frenchie)
Donald Ewing - Skip (preceded surname in place of Donald)
Clyde Foley - Red (preceded surname in place of Clyde)
William Frizzell - Lefty (preceded surname in place of William)
Don Gibson - The Sad Poet
Vern Gosdin - The Voice
Jack Greene - The Jolly Greene Giant
Merle Haggard - Hag
Tom T. Hall - The Storyteller
Harold Hawkins - Hawkshaw, Eleven Yards of Personality, Eleven and One Half Yards of Personality, Eleven Yards of Personality and a Song
Johnny Horton - The Singing Fisherman
Goldie Hill - The Golden Hillbilly
Wanda Jackson - Queen of Rockabilly
Waylon Albright Jennings - Shooter
Waylon Jennings - Hoss, Chief, Waymore
George Jones - Possum, No Show Jones
Louis Marshall Jones - Grandpa (preceded surname in place of Louis Marshall)
Doug Kershaw - The Ragin' Cajun
Jerry Lee Lewis - Killer
Loretta Lynn - The Coal Miner's Daughter
David Macon - Uncle (preceded his first name), Dixie Dewdrop
Bill Monroe - Father of Bluegrass
Aubrey Mullican - Moon (preceded surname in place of Aubrey), King of the Hillbilly Piano Players
Willie Nelson -The Red-Headed Stranger, Shotgun Willie
Thomas 'Tommy' Overstreet- T. O., what some D. J.'s reportedly called him.
Alvis Owens, Jr. -Buck. As the story goes, Buck was the name of a donkey on the Owens farm. When Alvis, Jr. was around 3 or 4 years old he walked into the house and announced that his name also was “Buck”. That was just fine with the family and from then on Alvis Owens, Jr. was called “Buck”.
Jim Reeves - Gentleman Jim
Charlie Rich - The Silver Fox
Nolan Rinehart - Cowboy Slim (One of the first singing cowboys)
James “Jimmie” Rodgers - The Singing Brakeman, America's Blue Yodeler, The Father of Country Music
Kenny Rogers - The Gambler
Joe Simpson - Red (preceded surname in place of Joe)
Carl Smith - Mister Country Music
Clarence Snow - Hank (preceded his surname in place of Clarence) , The Yodeling Ranger, The Singing Ranger
Woodrow Sovine - Red (preceded surname in place of Woodrow)
Russell Steagall - Red (preceded surname in place of Russell)
Gary Stewart - King of Honky-Tonk
George Strait - King George, King of Country Music
Ernest Tubb - The Texas Troubadour
Conway Twitty - High Priest of Country Music
Porter Wagoner - Mr. Grand Ole Opry
Billy Walker - The Tall Texan
Jerry Jeff Walker - The Gypsy Songman
Ottis Whitman, Jr. - Slim (preceded surname in place of Ottis), The Smilin' Starduster
Don Williams - The Gentle Giant
Hank Williams, Jr. - Bocephus
Hank Williams, Sr. - The Hillbilly Shakespeare
Bob Wills - The King of Western Swing
Faron Young - The Hillbilly Heartthrob, The Singing Sheriff, The Young Sheriff
Sources
First off I googled “nicknames of country music artists”. I got recent artists more than past artists. I turned my hint inside out. I found at Wikipedia a list of country artists. I would look an artist up, one that had to be from far back, and read about them to see if they had a nickname.
This site was helpful- https://www.hillbilly-music.com/index.php
Pics gotten via search on Google