From Mental Floss:
Stanley Martin Lieber got his start writing comic books, but hoped to one day graduate to more serious literary work and wanted to save his real name for that. He wrote the kids’ stuff under the pen name Stan Lee and eventually took it as his legal name after achieving worldwide recognition as a comic book writer.
Ann Landers was the pseudonym for several women who wrote the column over the years. The name was created by the column’s original author, Ruth Crowley, who adopted it because she was already writing a newspaper column about child care and didn’t want readers confusing the two. She borrowed the name from a friend of her family, Bill Landers, and made an effort to keep her real identity a secret.
Joanne Rowling’s publishers weren’t sure that the intended readers of the Harry Potter books—pre-adolescent boys—would would read stories about wizards written by a woman, so they asked her to use her initials on the book instead of her full name. Rowling didn’t have a middle name, though, and had to borrow one from her grandmother Kathleen to get her pen name J.K. Rowling.
When Eric Arthur Blair was getting ready to publish his first book, Down and Out in Paris and London, he decided to use a pen name so his family wouldn’t be embarrassed by his time in poverty. He chose the name George Orwell to reflect his love of English tradition and landscape. George is the patron saint of England and the River Orwell, a popular sailing spot, was a place he loved to visit.