Bleach VI: We Want Our Baby Bach Ribs (Aizen pull)

[Mando Knight]

In order of likelyhood, there are at least 3 points at which Ichigo could've attained his Zanpakuto:

1. He inherited it from his father.
2. He gained it when Rukia gave him her powers
3. He gained it because of Urahara Kisuke, who planned his restoration as a Shinigami.

He gained it when Rukia gave him her powers (or possibly when Urahara restored him, depending), but the form of it was inherited from Isshin...

...although we haven't seen Zangetsu/hollow Ichigo since Final Getsuga, I'm assuming that Zangetsu is still Zangetsu, due to Ichigo's Bankai being still more-or-less the same.