FWIW DEPT: I extracted this quick reference list.

Roy Rogers; Trigger (aka Golden Cloud*)/Trigger, Jr.
Dale Evans; Pal/Buttermilk
Buffalo Bill and Isham
Gene Autry and Champion
Clayton "Lone Ranger" Moore; Silver
Jay "Tonto" Silverheels; Scout
William "Hopalong Cassidy" Boyd; Topper
Little Beaver (Bobby Blake); Papoose)
Johnny Mack Brown; Rebel
Smiley Burnette; Ring Eye Nellie
Johnny Mack Brown; Rebel
Smiley Burnette; Ring Eye Nellie
Pat Butram; Nellie Belle [jeep]
Bob Baker; Apache
Bill "Radio" Boyd; Texas
Sunset Carson; Silver
Eddie Dean; Copper
Tim Holt; Duke
Bob Livingston; Starlight
Ken Maynard; Tarzan
Tom Mix; Tony Jr.
Jack Perrin; Starlight
Reb Russell; Rebel
Fred Thompson; Silver King
Jimmy Wakely; Sonny
John Wayne; Duke
Fred Scott; White Cloud.
Bob Allen; Pal
Rex Allen; Koko
Slim Andrews; Josephine
Tom Baker; Baron
Smith Ballew; Sheik
Jim Bannon; Thunder
Don "Red" Barry; Cyclone/Thunder
Rod Cameron; Knight
Leo "Pancho" Carrillo; Loco
Sunset Carson; Silver/Cactus Jr.
Andy Clyde; Johnny
Bill Cody; Chico
Don Coleman; Ghost
Buster Crabbe; Falcon
Gail "Annie Oakley" Davis; Target
Eddie Dean; Copper/Flash/White Cloud
Andy Devine; Joker
Wild "Bill" Elliott; Thunder/Sonny
Dick Foran; Smoky
Hoot Gibson; Starlight/Mud/Goldie/Silver/Pal
Monte Hale; Pardner
William S. Hart; Fritz
Jack Holt; Robin Hood
Tim Holt; Duke
Jack Hoxie; Scout
Buck Jones; Silver
Tom Keene; Rusty/Prince/Flash
Fuzzy Knight; Old Brownie
Allan "Rocky" Lane; Black Jack/Banner/Feather/Thunder
Lash LaRue; Rush/Black Diamond
Bob Livingston; Shamrock/Starlight
Jack Luden; Pal
Guy "Wild Bill Hickok" Madison; Buckshot
Jock "Range Rider" Mahoney/RAWHIDE-TV; Buckskin
Ken Maynard; Tarzan
Kermit Maynard; Baron/Rocky
Tim McCoy; Starlight/Midnight
Tom Mix; Tony/Tony Jr./Old Blue
Pete Morrison; Lightning
George O'Brien; Mike
Nell O'Day; Shorty
Jack F. Perrin; Starlight
Slim Pickens; Dear John/Appaloosa
Jack Randall; Rusty
Duncan "Cisco Kid" Renaldo; Diablo
Tex Ritter; White Flash
Buddy Roosevelt; Pardner
Reb Russell; Rebel
Charles "Durango Kid" Starrett; Raider
Bob Steele; Zane/Boy
Fred Thompson; Silver King
Tom Tyler; Ace/Baron
Jimmy Wakely; Sunny/Lucky/Sunset
John Wayne; Duke
Whip Wilson; Silver Bullet/Lightning
Fred Scott; White Dust
Olivia De Havilland; Golden Cloud* (before Roy Rogers)

Please ignore any errors!