Harry Potter and friends take quite a few trips to the so-called Forbidden Forest in the novels, but that doesn't mean we know all of its secrets.
The Harry Potter books and movies left people wanting to know more despite both having run for ten years each. Details derived from these mediums are common knowledge, and they are considered canon in their own right. However, there have been considerable facts delivered to fans through other outlets such as lexicons, video games, and JK Rowling herself, which people don’t strictly consider as “Harry Potter sources” since they weren’t found in easy-to-find materials like books and films. Still, these are canon regardless.