User blog comment:Xilinoc/Ch. 540 - The Sword Five/@comment-8295743-20130605172815/@comment-6963992-20130605174920

The hollow that attacked Masaki was able to bond with Ichigo's shinigami powers, creating his Zanpakuto and was basically a stand in for his Asuachi (due to Aizen's creations having a similar composition to the Asauchis). Now that Ichigo is getting a real one, that hollow was pulled out of him to become a real Zanpakuto. Zangetsu was actually the materialization of his quincy powers of a younger Juha Bach from 1000 yrs. ago which were originally one with his hollow/shinigami powers and dormant.