Bakemonogatari/WMG

Oshino is the Medicine Seller in A Form We'd Be Comfortable With.
Okay, honestly, I only started thinking this because I noticed Takahiro Sakurai voices both, but it's not that out there, is it? It's been noted that both shows have similar formats and concepts.

The Medicine Seller is immortal, and if he is a mononoke like on Mononoke's WMG, who says he can't shapeshift? He didn't bother back in the 1920s because while his outfit was rather out of place, there were still people running around in kimono back then, so nobody made a huge deal out of it. Nowadays everybody would call him out on it, so he traded in the ridiculous kimono for a ridiculous Aloha shirt. As for whatever happened to his sword...perhaps it became corrupt someway or another and became, who has occasional access to an absurdly sharp sword.

Shinobu is/was a
Well, she was a vampire. And in the first episode, during the Fake-Out Opening I swear, there was a priest and a few other things. I haven't read the light novel, but maybe an attempt to seal her by a burial agent? As for Araragi, his case was like Satsuki, where he was one of the 1 in 10,000 able to become a vampire and also able to skip the first two steps.
 * Type-Moon vampirism is permanent.
 * Indeed it is, but nothing is impossible for the Medicine Seller (see above).