http://www.starcraft2.com/screenshot.xml?s=100
Towards the bottom center. They can be neither Protoss or Zerg, but bear remarkable similarities to both. Anyone who's done the secret zerg mission will know exactly what I'm talking about 😉
Fourth Race in Starcraft 2?
I do remember that, but Why can't they be a Zerg unit they haven't revealed yet?
EDIT: HAHA I GET FAIL POINTS. Those ARE Zerg
(IIRC) there IS a 4th race in Starcraft 2. It's just not a playable faction. (read: no campaign for them).
EDIT: btw, the whole screenshots section of the StarCraft 2 website is offline.