I still have no idea how they spawn, but the latest ptr patch began filling my ptr pet journal with random pets. I was lucky enough to get a poor quality Val'kyr, which I was able to stone.
It appears to be H/P and as others have said, it looks like a dwarf female. When summoned, it makes various moaning noises if you click it.
It's 3rd ability-- Haunt-- is rather odd. It kills the Val'kyr, dotting your opponent for 4 rounds. If your opponent's pet is alive after 4 rounds, your Val'kyr returns to life, but if your opponent's pet dies, your Val'kyr stays dead. If your other pets are dead and Val'kyr casts Haunt, your opponent wins.
Overall, I don't see how the Unborn Val'kyr is better than a Skull, but if you don't mind the mini female dwarf look it's a pretty pet to summon.