well i think that it is Reg. Black cause in the Order of the Pheonix then they were going through Sirius's house -Now Harry's- and they found a Locket that they could not open.... maybe that is the Horocrux.... and Harry just has to go and find it now... although i cant recall did they throw it out or not... if not then maybe it is still there -or Mundugs took it?
All because it is a locket that they could not open, that doesn't mean that it is one of the Horcruxes! Sirius's mum could've put a charm on it, like she did to her picture. I doubt that bloke stole the locket, even if it is the horcrux, the Dark Lord would never be able to find it. Especially when Dumbledore is dead. the secret of the location of the Order of the Phoenix remains with him in his tomb. And no, I'm not saying that's a good thing either, just in case if someone was thinking that. 🙂