It's just a head, not a skull, but an actual head. It's dead already, but he head keeps moving like it's alive.

Glitch i imagine, what platform are you on? You can get rid of it on pc through the console.

strike the head for a free dragon soul!

Congratulation, you just ran into the extra rare mole-dragon!

Smacking it around, I can't absorb the soul.. I found it that way and had nothing to do with killing it.
I think its spawn glitched and it died.