The Dalton Brothers were originally introduced in Lucky Luke #6 and modeled after the real Dalton Brothers: Bob, Grat, Bill and Emmett. The Daltons were killed in the end of that story, but since they proved very popular by readers, writer/artist Morris introduced their cousins Joe, William, Jack and Averell in issue #12. They have the same appearance as the original Dalton brothers, but are less dangerous.
Morris, talking about The Daltons:
What got me interested in George Marshall's film, "When The Daltons Rode", is that the four brothers were united to defend evil causes. They really existed but, of course, they weren't twins and they didn't have staggered height. They were cousins of the James brothers, and liked to believe they were crueler than them. Actually, the were real idiots. The planned minutely holds up that didn't bring them any loot in. While I was in New York, I did a lot of research about their lives. Killing them in the end of "Hors-la-loi" (issue #6, "Outlaws") was a big blooper. I received many letters of readers who saw them as very funny characters, asking me to put them back. I created then the cousins, even dumber, because I noticed that stupidity and evil goes very well together and gives rise to hilarious jokes.
Other members of the Dalton clan are:
Ma Dalton: She is far more dreadful than her sons. She often hides a file inside a piece of bread to help them getting out of prison. Averell, the tallest, normally tries to eat the bread before getting the file out. Sometimes, the bread is even harder than the file and they use it to file the prison bars.
Father Dalton: He died the day he decided to open safes using dynamite instead of his tools.
Oncle Dalton: He is the black sheep of the family, and lives in Switzerland as a successful banker.