The Answers

A F James, Alexander Franklin James, nicknamed "Buck" by his family and associates.
                                          
D A Mather, Dave Allan Mather, nicknamed "Mysterious Dave" Mather. A some-time lawman and sometime-outlaw.
                                          
W B Masterson, William Barclay Masterson, he was given the name Bartholomew when he was born, hence his nickname "Bat." Changed his name when he grew older.
                                       
W H McCarty, William Henry McCarty, alias William Antrim, alias Henry Antrim, alias William Bonney. Know as "Billy the Kid", he was born in Indiana or New York City or Cork,Ireland. His mother married a guy named Antrim, hence part of the confusion. He fought in the "Lincoln County War", killed around six (not twenty-one!) men, rustled cattle, and was killed by Pat Garrett.
                                       
J H Holliday, John Henry Holliday, a dentist, he went West because of his tuberculosis (gee, a tubercular dentist coughing and working on your teeth -- what a charming picture!). Nicknamed "Doc", of course.
                                       
W M Tilghman, William Matthew Tilgham, nicknamed "Bill." One of the better lawmen, and he stayed on the side of the law.
                                       
J A Slade, Joseph Alfred Slade, nicknamed "Jack". Born in Illinois, a great guy when sober and a mean, mean drunk, he shot Jules Beni to pieces over three days just because Beni had shot him in the stomach with a sawed-off shotgun. Hanged by vigilantes in Monatana. Mark Twain met him (see "Roughing It" for the incident).
                                          
J B Hickok, James Butler Hickok, born in Illinois, nicknamed "Duck Bill" by his associates because of his protruding upper lip -- the reason he grew a mustache. Nickname changed to "Wild Bill" by Ned Buntline for the dime novels; shot and killed in a poker game in Deadwood, Dakota Territory. NOT the husband of Calamity Jane, no matter what she said.
                                          
J W James, Jesse Woodson James, nicknamed "Dingus" by his family and associates. He didn't swear, but made up the word "Dingus!" instead, hence the nickname. DNA analysis in 1995 determined that there is a 99.9% chance that he is buried in the grave of Jesse James.
                                          
C E Boles, Charles Earl Boles, nicknamed "Black Bart", a stage robber who never harmed anyone except the Wells Fargo company. Little authoritative is known about him, except that he had a farm near Decatur, Illinois, and left his wife and children there when he went to California (they never saw him again). Name confirmed from the rolls of the 116th Illinois Volunteers, in which he served during the US Civil War.
                                          
T E Ketchum, Thomas Edward Ketchum, nicknamed "Blackjack". A miserable excuse of a train robber, when he was hung in New Mexico in 1901 the hangman botched the job and ripped Ketchum's head from his body. He did say, "I'll be in hell before you'll be at breakfast" to his executioners.
                                          
R L Parker, Robert Leroy Parker, a good Mormon boy who took the name "Butch Cassidy" after a much less famous outlaw he once rode with. There is no record that he killed anyone. Heck, you've seen the movie!
                                          
J R Smith II, Jefferson Randolph Smith II, nicknamed "Soapy" from the con he ran in Denver. Later, in Skagway he pretty much ran the town during the Days of '98 and was shot to death by irate citizens.
                                       
O P Rockwell, Orrin Porter Rockwell, known as "Old Port." Born in Vermont he was Joseph Smith's and later Brigham Young's personal bodyguard. Died with his boots on, but considered one of the most implacable killers in the Old West -- but only when told to do so.
                                       
T I Courtright, Timothy Isaiah Courtwright, nickname "Long Hair Jim." Sometimes a lawman, he was killed by Luke Short when a lucky shot by Short took off Courtright's right thumb. While Courtright tried to do a "Border Shift" to move his gun to his left hand, Short shot him five more times.

Sources:: Various books, including but not limited to, Cunningham's "Triggernometry"; "Outlaws of the Pacific Northwest", Gulick; "Badasses of the the Old West", Erin Turner, ed.; "Knights of the Green Cloth", DeArment; "Old Deadwood Days", Bennett; "Shooters", Metz.      

I include some of the sources because a lot of these have a lot of controversy.