There is a vendor in PE that does allow you to exchange for one of these - it was put there during a promotion where a version of the mount with purple flames was available and the vendor was added so people who had that and wanted the original could trade for it.
Now the mount itself can be acquired from the Tradebar store or Nightmare lockboxes (which are fairly rare) or on the Auction House.
If you don't mind the lack of armor plates and you already have an epic mount in your stable, get the green quality Inferno Nightmare for the skin. Should be dirt cheap.
"We have always been at war with Dread Vault" ~ Little Brother
Comments
Now the mount itself can be acquired from the Tradebar store or Nightmare lockboxes (which are fairly rare) or on the Auction House.
Winter Lily (CW) / Winter Rose (DC) / Winter Ivy (HR)
Pandora's Misfits Guild Leader