[Update] And the answer is: Spirit Beast! More details at the end of the post.
Skoll is … well, as you can see here, he’s an ice-blue worg with built-in lightning bolts which are constantly flickering about his body. (Even with the model viewer it was hard to get a pic that does justice to the lightning.)
We don’t have a lot of information on this creature yet. Skoll is described as a neutral (yellow dot) tameable rare. Unfortunately the only in-game screenshot shows him dead already so we don’t have confirmation that he can be tamed.
And assuming that he can be tamed, we also don’t know for certain which pet family he might be in. Yes, he is clearly a saberworg … but he is also a neutral rare beast with a spectral look and built-in lightning. It’s not beyond the realm of possibility that he qualifies as our first non-feline Spirit Beast.
[Update] Commenter Safrienaer (aka Nanyana on the PTR) has tamed one of these bad boys and confirms that they are spirit beasts with the skills Prowl and Spirit Strike. And yes, Skoll keeps his lightning effect when he’s tamed. (You can just see the lightning around his ears in Safrienaer’s pic.)