Voldemort is the only one to make seven, but surely at some point, other dark wizards would have tried/succeeded in making horcruxes. Are there any other wizards that have created Horcruxes?
Other Wizards have tried/succeeded in making Horcruxes, starting with Herpo the Foul
SU: Oh, Jo, but those "horcruxes", though, I tell you, they're so much to ask still about those, you know?... I mean, who, okay, we have to know. Who created the first Horcrux? Was it Grindelwald? Salazar? Who did that?
JKR: D'you know what, I've got a feeling it was Herpo, which is H-E-R-P-O.
SU: Herpo the Foul?
JKR: ... Herpo the Foul, exactly, yeah. Yeah. But you know wizards would've been looking for ways to do exactly what Voldemort did for years, and some of the ways they would've tried would've killed them, so I imagine it... well, there's huge parallels. Splitting the atom would be a very good parallel in our world. Something that people imagined might be able to be done, but couldn't quite bring it off, and then... and then people started doing it with sometimes catastrophic effects. So that's how I see the Horcrux.
SU: Right, because you said that Tom Riddle said there would've been, or Dumbledore did, somebody said that there was only one person--
JKR: Yeah, but I would imagine that other people, you know, other people are going to have tried. I think it would be naive not to think that people have been trying for a long time, and thought they succeeded and hadn't, or else, or else you know maim themselves or kill themselves in the attempt. It's such a dangerous thing to do.
PotterCast Interviews J.K. Rowling, part one