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.
Unborn Val'kyr on ptr
Re: Unborn Val'kyr on ptr
Seems to me the Valkyr would work well as a first pet. Curse of Doom, then Haunt. Switch to second pet whom uses sweep or deathgrip to knock the victim to the backrow. Where it will take severe damage, but likely not dying from it.