It was a vampire. I didn't realize this until I saw someone say it the comments on Amazon. But it makes complete sense:

1. It looked like Nosferatu (I kept saying that repeatedly while I was watching the movie but still didn't realize it was a vampire!)
2. It turned to stone/dust from light.
3. It attacked people. Could have been a zombie, but vampires do attack too.
4. Getting bitten by one turns you into one.
5. The thing the worker at the beginning first dug up might have been a stake - vampire had been staked; stake comes out so vampire is free.
6. When you see its hand in the ground when he first discovers it it appears to be wearing something that men wore centuries ago - so that fits with the ancient vampire accidentally unleashed on the world theme, which is common.

Seeing it as a vampire for me raises the rating of the movie from a 3 to a 4. It was good, but kind of got dull about halfway through. But realizing the thing was a vampire, it was very unique to portray a vampire in this way. So a whole extra point for that.