There's little information on where these come from, and judging from the state of the various spells involved, it's likely that they're simply not fully implemented yet. We can speculate, though! BB notes that Blizzard has said that there'll be at least one more patch before 4.0, and these could be a faction- or quest-based reward involving Icecrown or the Ashen Verdict.
I got a little antsy about making the iconic death knight amount available to the general public, if that's even happening, but then my brain turned on and I remembered that the forsaken, human, dwarf, tauren, night elf, blood elf, and troll mounts have been available to other races in one way for a very long time. Of course, no other class can get the paladin charger model, for example, so maybe I'm approaching it all wrong.
Either way. We'll likely see these bony ponies pop up again at a later date, and we'll keep you posted.