Which was the original word- "pock" or "pox"?

"Pock" is the older word. "Pox" is the variant of this word. According to etymonline.com:

Pock: O.E. pocc "pustule," from P.Gmc. *puh(h)- "to swell up, blow up" (cf. Du. pok, Low Ger. poche), from PIE base *bhu- "to swell, to blow." The plural form, M.E. pokkes, is the source of pox, which since early 14c. has been used in the sense "disease characterized by pocks.

Pox : late 15c., spelling alteration of pockes, pl. of pocke

As you can see, "pock" originated earlier, by about 100 years, and then there was a sudden shift to spell it 'x' instead of 'cks', due to unknown reasons to me, but "pock" is definitely older.