Sally Smith was one of the original forty students at Hogwarts that J.K. Rowling created. She never appeared in canon, but did appear in the BBC interview with J.K. Rowling in 2001, Harry Potter and Me. Her name also appears in a new writing from J.K. Rowling on Pottermore, The Original Forty. (HPM) (Pm)
From the Web
Click here to learn more about the 2001 BBC interview with J.K. Rowling, Harry Potter and Me.
Click here to read J.K. Rowling's new writing on Pottermore about the original forty.
This source provides a good analysis of what happened to the original forty.