These five people were wizards or witches (or both) who were known to be registered Animagi; however their animal forms are unknown. Hermione Granger referred to them by saying that there were only seven registered Animagi in the 20th century. Since the Marauders trio (Prongs, Padfoot and Wormtail) were unregistered, it is at least conceivable that others may have effected this transformation without registering.