Some Fel Lords also look like they have armoring on their backs too and some more looks like bone spikes. That guy was from the cinematic so I just took it as a random Fel Guard.
The new pic of the clean metal looking back spikes, those guys look like Eredar that went down the Vengeance path.
I dont think we're ever channeling one race, because how can you channel a race we never met? DreadLords, Doomguards and succubi all have the same Havoc appearances we gain aswell. What you see when we transform, that's the Elven race going Havoc or Vengeance, but only temporarily. I imagine that Humans, Orcs and Tauren wouldn't use the same model either if they did it too.