There's also a chance that it's a rogue wizard or witch who has a personal vendetta against the Malfoy family. Maybe someone who was wronged by Lucius in the past and now wants to make the family suffer by taking Draco. This person could be hiding in the Forbidden Forest or some other remote location, waiting for the right time to make their demands known.
Another option is that it's a group of students from other schools. Maybe from Durmstrang or Beauxbatons. They could be jealous of Hogwarts' reputation and think that by kidnapping Draco, a well - known Hogwarts student, they can get some sort of advantage or revenge for past slights in the Triwizard Tournament or other inter - school competitions.
It could be some of the more radical pure - blood supremacists. They might see Draco as a symbol of the old pure - blood families that are starting to lose their influence. By kidnapping him, they hope to send a message to the Malfoy family and other pure - bloods that they need to be more aggressive in maintaining their status.