I agree with Werns although maybe Mundungus stole the locket and a)still has it or b) sold it. I also like the idea of it being someone other that Regulus but I really believe it is him. I think he got in too deep with Voldemort and decided he wanted out but by then it was too late. I think Voldemort entrusted each Horcrux to a different (well trusted) Death Eater and instructed them on how and where to hide and protect it. I don't know if he actually gave one to Regulus but if he didn't, maybe Regulus overheard him telling another DE what to do. Then Regulus took Kreacher to steal the Horcrux and try and defeat LV.