J.K. Rowling’s original surname was Rowling.
She was born Joanne Rowling, so “Rowling” was her family surname before she became famous. The name J.K. Rowling appeared on the Harry Potter books because her publisher wanted a name that would not clearly signal the author’s gender to young male readers. Rowling did not have a middle name at birth, so the K was added for Kathleen, the name of a paternal grandmother.
After marrying Neil Murray in 2001, Rowling sometimes used Joanne Murray in personal and legal contexts. Her books continued to be published as J.K. Rowling, and that is the name most associated with her public career.
The later pen name Robert Galbraith is different: it was used for her crime novels to separate them from the Harry Potter and adult-fiction readership. “Joanne Kathleen Rowling” is therefore a constructed form of her name, while Rowling itself is her original family surname.